[tunnelbroker-client] Adding publish_message client function
Summary:
This diff introduces a publish_message client function for the shared Tunnelbroker client library.
The function handles publishing message payload to the gRPC stream which is bind to the mpsc tx argument.
Linear task: ENG-2728
Test Plan:
- Keyserver service is successfully built.
- The function is used in the following D6460 in a NAPI-RS bridge function
Reviewers: varun, jon, bartek
Reviewed By: varun, jon
Subscribers: ashoat, tomek, atul
Differential Revision: https://phab.comm.dev/D6284