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

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