[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