[Yang] Add resource_type and downstream_subrole to yang model (#9696)

#### Why I did it
As a fix for https://github.com/Azure/sonic-buildimage/issues/9574

#### How I did it
Enhance yang model for networking-metadata

#### How to verify it
Unit testing

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

@@ -60,6 +60,12 @@
     },
     "DEVICE_METADATA_CORRECT_CLOUDTYPE_REGION_CONFIG": {
         "desc": "Verifying cloudtype and region configuration."
+    },
+    "DEVICE_METADATA_DOWNSTREAM_SUBROLE_CONFIG": {
+        "desc": "Verifying downstream subrole configuration."
+    },
+    "DEVICE_METADATA_RESOURCE_TYPE_CONFIG": {
+        "desc": "Verifying resource type configuration."
     },
 	"DEVICE_METADATA_VALID_SUBTYPE": {
 		"desc": "Verifying valid subtype value"

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

@@ -160,6 +160,24 @@
             }
         }
     },
+    "DEVICE_METADATA_DOWNSTREAM_SUBROLE_CONFIG": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "downstream_subrole": "downstream_subrole_y"
+                }
+            }
+        }
+    },
+    "DEVICE_METADATA_RESOURCE_TYPE_CONFIG": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "resource_type": "resource_type_x"
+                }
+            }
+        }
+    },
     "DEVICE_METADATA_VALID_SUBTYPE": {
         "sonic-device_metadata:sonic-device_metadata": {
             "sonic-device_metadata:DEVICE_METADATA": {

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

@@ -127,6 +127,14 @@ module sonic-device_metadata {
                     type string;
                 }
 
+                leaf downstream_subrole {
+                    type string;
+                }
+
+                leaf resource_type {
+                    type string;
+                }
+
                 leaf subtype {
                     type string {
                         pattern "DualToR";