Fix swagger issue: add missing etag (#14434)

Author: jiacheng-L

specification/keyvault/resource-manager/Microsoft.KeyVault/preview/2020-04-01-preview/keyvault.json

@@ -1783,6 +1783,10 @@
           "type": "string",
           "description": "Id of private endpoint connection."
         },
+        "etag": {
+          "type": "string",
+          "description": "Modified whenever there is a change in the state of private endpoint connection."
+        },
         "properties": {
           "$ref": "#/definitions/PrivateEndpointConnectionProperties",
           "x-ms-client-flatten": true,