[yang]: Update DEVICE_METADATA yang models to support 'sub_role' (#10161)

#### Why I did it
Fix https://github.com/Azure/sonic-buildimage/issues/9591
#### How I did it
Add 'sub_role' to device_metadata yang models.
#### How to verify it
Run UT for sonc-yang-models.

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -302,7 +302,8 @@
                 "asic_name": "Asic0",
                 "switch_id": "2",
                 "switch_type": "voq",
-                "max_cores": "8"
+                "max_cores": "8",
+                "sub_role": "FrondEnd"
             }
         },
         "VLAN": {

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

@@ -100,6 +100,9 @@
     },
     "DEVICE_METADATA_CORRECT_VOQ_CONFIG": {
         "desc": "Verifying VOQ configuration."
-     }
+    },
+    "DEVICE_METADATA_VALID_SUB_ROLE_CONFIG": {
+        "desc": "Verifying valid sub_role configuration."
+    }
 
 }

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

@@ -274,5 +274,14 @@
                 }
             }
         }
+    },
+    "DEVICE_METADATA_VALID_SUB_ROLE_CONFIG": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "sub_role": "FrontEnd"
+                }
+            }
+        }
     }
 }

src/sonic-yang-models/yang-models/sonic-device_metadata.yang

@@ -127,6 +127,11 @@ module sonic-device_metadata {
                     type string;
                 }
 
+                leaf sub_role {
+                    type string;
+                    description "sub_role indicates if ASIC is FrondEnd or BackEnd.";
+                }
+
                 leaf downstream_subrole {
                     type string;
                 }