[Identity] Add client device service to identity service

Summary:
This adds a stubbed out version of the identity service
given the current state of the protobuf file.

This is intended to allow for the RPCs be worked in parallel.

Part of https://linear.app/comm/issue/ENG-3347

Test Plan:

cd services/identity
cargo run -- server

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7130