tfdbg: change grpc protocol to make it possible to switch on/off gated_grpc...

tfdbg: change grpc protocol to make it possible to switch on/off gated_grpc debug ops at the first Session.run()

Previously, the request_watch() and request_unwatch() calls will not take effect the current Session.run() has ended. This CL eliminates that limitation by letting PublishCoreMeatadata() and PublishGraph() read an EventReply from the server before they return.

For the Python API, this change is backward compatible. The on_core_meatadata() and on_graph_def() methods can continue to return nothing (None), in which case the base server class will just replace the None with a default-constructor EventReply.

PiperOrigin-RevId: 168608272