[yang] Add anchor_route_source in DEVICE_METADATA (#22294)

* [yang] Add anchor_route_source in DEVICE_METADATA

* Update to listhy I did it
T2 local anchor support was added by this PR #21732
We plan to use a new filed to specified the source of anchor route

How I did it
Add new field in DEVICE_METADAT

How to verify it
UT

Author: yaqiangz

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

@@ -211,9 +211,12 @@
     },
     "DEVICE_METADATA_VALID_T2_GROUP_ASNS": {
         "desc": "Verifying valid t2_group_asns."
-     },
+    },
      "DEVICE_METADATA_VALID_T2_GROUP_ASNS_INVALID": {
          "desc": "Verifying invalid t2_group_asns.",
          "eStrKey": "InvalidValue"
-      }
+    },
+    "DEVICE_METADATA_VALID_ANCHOR_ROUTE_SOURCE": {
+        "desc": "Verifying valid anchor_route_source."
+    }
 }

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

@@ -621,5 +621,14 @@
                 }
             }
         }
+    },
+    "DEVICE_METADATA_VALID_ANCHOR_ROUTE_SOURCE": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "anchor_route_source": ["local"]
+                }
+            }
+        }
     }
 }

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

@@ -295,6 +295,10 @@ module sonic-device_metadata {
                     description "ASNs inner same group";
                 }
 
+                leaf-list anchor_route_source {
+                    type string;
+                    description "Set source of anchor route";
+                }
             }
             /* end of container localhost */
         }