[identity] return user ID on login

Summary:
if a user logs in on a new client, they will need their user ID

Depends on D7536

Test Plan: tested later as part of OPAQUE login workflow

Reviewers: jon, bartek, ashoat

Reviewed By: jon, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7537