[commtest] use shared/grpc_client for identity clients
Summary:
remove all custom clients and replace them with the grpc_clients library clients
Depends on D9471
Test Plan: ran cargo test and all tests but an unrelated backup integration test passed (wasn't running backup service)
Reviewers: bartek, michal, jon
Reviewed By: bartek
Subscribers: ashoat, tomek, wyilio
Differential Revision: https://phab.comm.dev/D9472