[Identity] Add client <-> identity protobuf defintions

Summary:
This meant to get consensus on how a device should
communicate with the identity service.

https://linear.app/comm/issue/ENG-3264

Test Plan: N/A, this is more about the design of the service than testing

Reviewers: varun, bartek, ashoat

Reviewed By: varun, ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7003