[grpc-clients] Add helpers for services grpc client

Summary:
Added helpers to instantiate Identity auth client with service-to-service token, similiar to create_auth_client() for CSAT requests

Depends on D14465

Test Plan: cargo build. Client is used in the next diff

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14466