[identity] update change password RPCs to match whitepaper

Summary: per the whitepaper, we want clients to run Password-based Authentication before running Password-based Registration with the new password.

Test Plan: tested later in stack by calling the new RPCs from web and native

Reviewers: ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12465