[identity] register user finish

Summary:
Implement RegisterPasswordUserFinish RPC.

We use the cached state from RegisterPasswordUserStart to complete the
OPAQUE registration workflow, storing a password file for the user and returning
an access token and user ID

Depends on D7533

Test Plan: called the start and finish RPCs, got back a token and id, and verified that the DDB tables were updated. also tried calling the finish RPC >10s after start and got back an error, as expected

Reviewers: jon, bartek, ashoat

Reviewed By: jon, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7534