Added new virtualNetwork property to load balancer backend address pool (Azure#21436)

Author: irrogozh

specification/network/resource-manager/Microsoft.Network/stable/2022-07-01/loadBalancer.json

@@ -1824,6 +1824,10 @@
           "type": "integer",
           "format": "int32",
           "description": "Amount of seconds Load Balancer waits for before sending RESET to client and backend address."
+        },
+        "virtualNetwork": {
+          "$ref": "./network.json#/definitions/SubResource",
+          "description": "A reference to a virtual network."
         }
       },
       "description": "Properties of the backend address pool."