message: add type_url to DeltaDiscoveryResponse
  Risk Level: none, adding unused proto field
diff: @@ -83,9 +83,8 @@ message DiscoveryResponse {
  // validated via a dry run.
  bool canary = 3;
  - // Type URL for resources. This must be consistent with the type_url in the
  - // Any messages for resources if resources is non-empty. This effectively
  - // identifies the xDS API when muxing over ADS.
  + // Type URL for resources. Identifies the xDS API when muxing over ADS.
  + // Must be consistent with the type_url in the 'resources' repeated Any (if non-empty).
  string type_url = 4;
  // For gRPC based subscriptions, the nonce provides a way to explicitly ack a
  @@ -191,6 +190,12 @@ message DeltaDiscoveryResponse {
  // in the DeltaDiscoveryRequest.
  repeated Resource resources = 2 [(gogoproto.nullable) = false];
  + // field id 3 IS available!
  +
  + // Type URL for resources. Identifies the xDS API when muxing over ADS.
  + // Must be consistent with the type_url in the Any within 'resources' if 'resources' is non-empty.
  + string type_url = 4;
  +
  // Resources names of resources that have be deleted and to be removed from the xDS Client.
  // Removed resources for missing resources can be ignored.
  repeated string removed_resources = 6;
  