Add mgmt_type into device metadata YANG model.

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#### Why I did it

Enhance yang model for networking-metadata.

Adding a new field in device metadata YANG model to distinguish the management type of the device. Depends on the users, this field can be used to tell the purpose of deployment or who/how the device is being managed.

##### Work item tracking
- Microsoft ADO **(number only)**:

#### How I did it

This change adds a new field called mgmt_type for this purpose.

#### How to verify it

Unit test.

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Author: mssonicbld

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

@@ -88,6 +88,9 @@
     "DEVICE_METADATA_RESOURCE_TYPE_CONFIG": {
         "desc": "Verifying resource type configuration."
     },
+    "DEVICE_METADATA_MGMT_TYPE_CONFIG": {
+        "desc": "Verifying mgmt type configuration."
+    },
     "DEVICE_METADATA_VALID_CLUSTER": {
         "desc": "Verifying valid cluster configuration."
     },

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

@@ -237,6 +237,15 @@
             }
         }
     },
+    "DEVICE_METADATA_MGMT_TYPE_CONFIG": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "mgmt_type": "mgmt_type_x"
+                }
+            }
+        }
+    },
     "DEVICE_METADATA_VALID_CLUSTER": {
         "sonic-device_metadata:sonic-device_metadata": {
             "sonic-device_metadata:DEVICE_METADATA": {

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

@@ -157,6 +157,11 @@ module sonic-device_metadata {
                     type string;
                 }
 
+                leaf mgmt_type {
+                    type string;
+                    description "Indicates the management type of this device.";
+                }
+
                 leaf cluster {
                     type string;
                     description "The switch is a member of this cluster.";