[identity] Make authenticated X3DH RPCs use userID

Summary:
Main part of ENG-5659. This diff makes it happen with minimal changes to the existing code.

Depends on D9957

Test Plan: Called the RPC with a userID and it worked as expected. Compared the results with the old username RPCs in unauth proto.

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10048