[yang] Update YANG model for mirror session to support decimal value for GRE type (#10140)

#### Why I did it
PR  https://github.com/Azure/sonic-utilities/pull/1825 added validation for the input of `config mirror session add`, and only decimal value is accepted.
An issue https://github.com/Azure/sonic-buildimage/issues/10096 was raised to suggest accepting HEX value as well, and the suggestion makes sense to me.

To accept HEX value for GRE type, and keep backward compatibility as well, I updated the YANG model to support both decimal and hexadecimal input for GRE type.

#### How I did it
Update the regex for GRE type.

#### How to verify it
Verified by UT
```
platform linux -- Python 3.9.2, pytest-6.0.2, py-1.10.0, pluggy-0.13.0
rootdir: /sonic/src/sonic-yang-models
plugins: pyfakefs-4.5.4, cov-2.10.1
collected 3 items                                                                                                                                                                                     

tests/test_sonic_yang_models.py ..                                                                                                                                                              [ 66%]
tests/yang_model_tests/test_yang_model.py .                                                                                                                                                     [100%]

========================================================================================== 3 passed in 2.53s ==========================================================================================
```

#### Description for the changelog
Update YANG model for mirror session to support decimal value for GRE type.

src/sonic-yang-models/tests/yang_model_tests/tests/mirror_session.json

@@ -1,6 +1,18 @@
 {
-    "MIRROR_ERSPAN_ENTRY_WITH_VALID_VALUES": {
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_HEX_VALUES": {
-        "desc": "Configuring ERSPAN entry with valid values."
+        "desc": "Configuring ERSPAN entry with valid heximal values."
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_HEX_VALUES_1": {
+        "desc": "Configuring ERSPAN entry with valid heximal values."
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_DEC_VALUES": {
+        "desc": "Configuring ERSPAN entry with valid decimal values."
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_DEC_VALUES_1": {
+        "desc": "Configuring ERSPAN entry with valid decimal values."
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_DEC_VALUES_2": {
+        "desc": "Configuring ERSPAN entry with valid decimal values."
     },
     "MIRROR_ERSPAN_ENTRY_WRONG_TYPE": {
         "desc": "Configuring ERSPAN entry with invalid type",
@@ -26,6 +38,14 @@
         "desc": "Configuring ERSPAN entry with invalid GRE type",
         "eStrKey" : "Pattern"
     },
+    "MIRROR_ERSPAN_ENTRY_WRONG_GRE_TYPE_1": {
+        "desc": "Configuring ERSPAN entry with invalid GRE type",
+        "eStrKey" : "Pattern"
+    },
+    "MIRROR_ERSPAN_ENTRY_WRONG_GRE_TYPE_2": {
+        "desc": "Configuring ERSPAN entry with invalid GRE type",
+        "eStrKey" : "Pattern"
+    },
     "MIRROR_ERSPAN_ENTRY_GRE_WRONG_TYPE": {
         "desc": "Configuring ERSPAN entry with invalid GRE type",
         "eStrKey" : "When"

src/sonic-yang-models/tests/yang_model_tests/tests_config/mirror_session.json

@@ -1,5 +1,5 @@
 {
-    "MIRROR_ERSPAN_ENTRY_WITH_VALID_VALUES": {
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_HEX_VALUES": {
         "sonic-mirror-session:sonic-mirror-session": {
             "MIRROR_SESSION": {
                 "MIRROR_SESSION_LIST": [
@@ -15,6 +15,70 @@
             }
         }
     },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_HEX_VALUES_1": {
+        "sonic-mirror-session:sonic-mirror-session": {
+            "MIRROR_SESSION": {
+                "MIRROR_SESSION_LIST": [
+                    {
+                        "name": "erspan",
+                        "type": "ERSPAN",
+                        "dst_ip": "11.1.1.1",
+                        "src_ip": "10.1.1.1",
+                        "gre_type": "0x0",
+                        "dscp": "10"
+                    }
+                ]
+            }
+        }
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_DEC_VALUES": {
+        "sonic-mirror-session:sonic-mirror-session": {
+            "MIRROR_SESSION": {
+                "MIRROR_SESSION_LIST": [
+                    {
+                        "name": "erspan",
+                        "type": "ERSPAN",
+                        "dst_ip": "11.1.1.1",
+                        "src_ip": "10.1.1.1",
+                        "gre_type": "1234",
+                        "dscp": "10"
+                    }
+                ]
+            }
+        }
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_DEC_VALUES_1": {
+        "sonic-mirror-session:sonic-mirror-session": {
+            "MIRROR_SESSION": {
+                "MIRROR_SESSION_LIST": [
+                    {
+                        "name": "erspan",
+                        "type": "ERSPAN",
+                        "dst_ip": "11.1.1.1",
+                        "src_ip": "10.1.1.1",
+                        "gre_type": "65535",
+                        "dscp": "10"
+                    }
+                ]
+            }
+        }
+    },
+    "MIRROR_ERSPAN_ENTRY_WITH_VALID_DEC_VALUES_2": {
+        "sonic-mirror-session:sonic-mirror-session": {
+            "MIRROR_SESSION": {
+                "MIRROR_SESSION_LIST": [
+                    {
+                        "name": "erspan",
+                        "type": "ERSPAN",
+                        "dst_ip": "11.1.1.1",
+                        "src_ip": "10.1.1.1",
+                        "gre_type": "0",
+                        "dscp": "10"
+                    }
+                ]
+            }
+        }
+    },
     "MIRROR_ERSPAN_ENTRY_WRONG_TYPE": {
         "sonic-mirror-session:sonic-mirror-session": {
             "MIRROR_SESSION": {
@@ -104,7 +168,39 @@
                         "type": "ERSPAN",
                         "dst_ip": "11.1.1.1",
                         "src_ip": "10.1.1.1",
-                        "gre_type": "0",
+                        "gre_type": "100000",
+                        "dscp": "10"
+                    }
+                ]
+            }
+        }
+    },
+    "MIRROR_ERSPAN_ENTRY_WRONG_GRE_TYPE_1": {
+        "sonic-mirror-session:sonic-mirror-session": {
+            "MIRROR_SESSION": {
+                "MIRROR_SESSION_LIST": [
+                    {
+                        "name": "erspan",
+                        "type": "ERSPAN",
+                        "dst_ip": "11.1.1.1",
+                        "src_ip": "10.1.1.1",
+                        "gre_type": "-1",
+                        "dscp": "10"
+                    }
+                ]
+            }
+        }
+    },
+    "MIRROR_ERSPAN_ENTRY_WRONG_GRE_TYPE_2": {
+        "sonic-mirror-session:sonic-mirror-session": {
+            "MIRROR_SESSION": {
+                "MIRROR_SESSION_LIST": [
+                    {
+                        "name": "erspan",
+                        "type": "ERSPAN",
+                        "dst_ip": "11.1.1.1",
+                        "src_ip": "10.1.1.1",
+                        "gre_type": "65536",
                         "dscp": "10"
                     }
                 ]

src/sonic-yang-models/yang-models/sonic-mirror-session.yang

@@ -99,7 +99,7 @@ module sonic-mirror-session {
                 leaf gre_type {
                     when "current()/../type = 'ERSPAN'";
                     type string {
-                        pattern "0[xX][0-9a-fA-F]*";
+                        pattern "0[xX][0-9a-fA-F]*|([0-9]|[1-5]?[0-9]{2,4}|6[1-4][0-9]{3}|65[1-4][0-9]{2}|655[1-2][0-9]|6553[0-5])";
                         length 1..6 {
                             error-message "Invalid GRE type";
                             error-app-tag gre-type-invalid;