[YANG][SNMP]: Add YANG model for SNMP_AGENT_ADDRESS_CONFIG table (#15587)

#### Why I did it
https://github.com/sonic-net/sonic-utilities/pull/472 Added SNMP_AGENT_ADDRESS_CONFIG table in config db.
This PR is to add corresponding YANG model for that table.
##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it
Added YANG modesl for SNMP_AGENT_ADDRESS_CONFIG.

keys: agent_ip, port number, vrf.
CLI implementaion checks if agent_ip, port number already exists in CONFIG_DB table, if it does, then new entry is not added.
So added another condition to ensure combination of agent_ip and port is unique.
Below is an example of how data looks like in DB:
```
127.0.0.1:6379[4]> HGETALL  "SNMP_AGENT_ADDRESS_CONFIG|10.1.1.1|161|foo"
1) "NULL"
2) "NULL"
127.0.0.1:6379[4]> HGETALL "SNMP_AGENT_ADDRESS_CONFIG|10.1.0.32|161|"
1) "NULL"
2) "NULL"
```
#### How to verify it
Added unit-test for various combinations and ensures that it passes.

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

@@ -113,5 +113,40 @@
     "SNMP_USER_PRIV_LONG_ENCRYPT_PASS_NEG_TEST": {
         "desc": "Load SNMP user with user type Priv with long encryption password",
         "eStrKey": "Range"
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG": {
+        "desc": "Load SNMP agent address config"
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_IPV6": {
+        "desc": "Load SNMP agent address config"
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_EMPTY_PORT_NUMBER": {
+        "desc": "Load SNMP agent address config with empty port number"
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_MGMT_VRF": {
+        "desc": "Load SNMP agent address config with mgmt vrf"
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_NO_VRF": {
+        "desc": "Load SNMP agent address config with no vrf",
+	"eStr": ["Missing required element"]
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_INVALID_PORT": {
+        "desc": "Load SNMP agent address config with invalid port",
+        "eStrKey": "InvalidValue"
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_DUPLICATE_IP_PORT": {
+        "desc": "Load two SNMP agent address config same ip and port",
+	"eStr": ["Unique data leaf(s)"]
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_INVALID_IPV4_ADDRESS": {
+        "desc": "Load SNMP agent address config with invalid IPv4 address",
+        "eStrKey": "InvalidValue",
+        "eStr": ["ip"]
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_INVALID_IPV6_ADDRESS": {
+        "desc": "Load SNMP agent address config with invalid IPV6 address",
+        "eStrKey": "InvalidValue",
+        "eStr": ["ip"]
     }
 }
+

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

@@ -502,5 +502,126 @@
                 ]
             }
          }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "10.0.0.1",
+                       "port": "161",
+                       "vrf_name": ""
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_IPV6": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "fd00::1",
+                       "port": "161",
+                       "vrf_name": ""
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_EMPTY_PORT_NUMBER": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "10.0.0.1",
+                       "port": "",
+                       "vrf_name": "mgmt"
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_MGMT_VRF": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "10.0.0.1",
+                       "port": "161",
+                       "vrf_name": "mgmt"
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_NO_VRF": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "10.0.0.1",
+                       "port": "161"
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_INVALID_PORT": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "10.0.0.1",
+		       "port": "65536",
+                       "vrf_name": "mgmt"
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_DUPLICATE_IP_PORT": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "10.0.0.1",
+                       "port": "161",
+                       "vrf_name": "mgmt"
+                    },
+                    {
+                       "agent_ip": "10.0.0.1",
+                       "port": "161",
+                       "vrf_name": ""
+                    } 
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_INVALID_IPV4_ADDRESS": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "340.1.1.10",
+                       "port": "161",
+                       "vrf_name": ""
+                    }
+                ]
+            }
+         }
+    },
+    "SNMP_AGENT_ADDRESS_CONFIG_INVALID_IPV6_ADDRESS": {
+        "sonic-snmp:sonic-snmp": {
+            "sonic-snmp:SNMP_AGENT_ADDRESS_CONFIG": {
+                "SNMP_AGENT_ADDRESS_LIST": [
+                    {
+                       "agent_ip": "2001:aa:aa:aa",
+                       "port": "161",
+                       "vrf_name": ""
+                    }
+                ]
+            }
+         }
     }
 }

src/sonic-yang-models/yang-models/sonic-snmp.yang

@@ -3,6 +3,12 @@ module sonic-snmp {
   prefix ssnmp;
   yang-version 1.1;
 
+  import ietf-inet-types {
+      prefix inet;
+  }
+  import sonic-vrf {
+    prefix vrf;
+  }
 
   organization
     "SONiC";
@@ -156,5 +162,42 @@ module sonic-snmp {
         }
       }
     }
+    container SNMP_AGENT_ADDRESS_CONFIG {
+      list SNMP_AGENT_ADDRESS_LIST {
+        key "agent_ip port vrf_name";
+        unique "agent_ip port";
+        description "List of SNMP agent listening IP Addresses and ports.";
+
+        leaf agent_ip {
+          type inet:ip-address;
+          description "SNMP agent listening IP";
+        }
+        leaf port {
+          type union {
+            type string {
+              pattern '';
+            }
+            type inet:port-number;
+          }
+          default "";
+          description "SNMP agent listening port number";
+        }
+        leaf vrf_name {
+          type union {
+            type string {
+              pattern '';
+            }
+            type string {
+              pattern 'mgmt';
+            }
+            type string {
+              pattern "Vrf[a-zA-Z0-9_-]+";
+            }
+          }
+          default "";
+          description "VRF name";
+        }
+      }
+    }
   }
 }