[keyserver] update registerUser function

Summary:
this function now uses comm-opaque2 to handle OPAQUE registration and follows the new API spec defined in identity_client.proto

this function can be deprecated once we've successfully double-written all users from ashoat's keyserver to the identity service

Depends on D7355

Test Plan: TBD, will rebase on jon's stack to test this later

Reviewers: ashoat, jon

Reviewed By: ashoat, jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7356