[identity] Move UpdateUserPassword RPCs

Summary:
Moved UpdateUserPassword RPC to the authenticated proto.

Depends on D9929

Test Plan: Registered with Identity Service, was able to log in. Hardcoded call to update_user_password() in native client and verified that password was updated by performning login again.

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9930