tfdbg: 1st step toward GRPC debug targets: Protobuf service definition

This CL defines the basic protocol buffer message and service that will be used in the communication between the debug ops and a debug server. The debug server is designed to run, e.g., inside the TensorBoard backend. It listens to intermediate tensor values (and potentially other information) encapsulated as Event protos published by the debug ops. The response message from the debug server is currently just an empty acknowledgment, but can hold fields if so desired in the future.
Change: 133191013