[identity] Scaffold RPC for device list updates

Summary:
I need only basic part of it for ENG-5425.
Proper implementation is tracked by ENG-5101.

Test Plan: Compiled Identity Service. Implementation and integration tests are done in subsequent diffs.

Reviewers: varun, will, ashoat, michal

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10797