[Tunnelbroker] generate client message ID for gRPC messages

Summary:
Issue: ENG-5149.

Protocol diagram: notion doc.

For gRPC calls we need to generate ID directly on Tunnelbroker to keep schema consistent.

Depends on D9459

Test Plan:
Send gRPC message and make sure it is properly persisted in DDB.
[ss]

Reviewers: michal, bartek, varun, jon

Reviewed By: michal, bartek

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9460