[identity] finish login

Summary:
finish OPAQUE login, return access token and user ID

Depends on D7538

Test Plan:
called both login RPCs with a previously registered user, got back
the existing user ID and a new access token

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7539