[Shared] Create shared grpc_client library

Summary:
Every service and native device will need to talk to either identity
service or tunnelbroker. This begins the trend with making identity service
and related protos available as a shared library.

https://linear.app/comm/issue/ENG-4640

Test Plan:

cargo build

This diff will be integrated in a later diff

Reviewers: bartek, varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8804