[protos][identity] Add RPC to find username by user ID

Summary:
Addresses ENG-7190.
Added RPC that, given user ID, retrieves username or wallet address

Depends on D11378

Test Plan:

• registered two users: one password user and one wallet user
• called the RPC for each, made sure that username is retrieved for the first, and wallet address + social proof pair is retrieved for the second

If the RPC format is approved, I'll add a commtest test too.

Reviewers: varun, ashoat

Reviewed By: varun, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11379