[identity] update user password

Summary:
updated_user_password start and finish methods. also added a DB method
to update the users table.

changed the .proto because we needed some more info to retrieve an access token, and we don't actually need to mint a new token after successful password update.

Depends on D7544

Test Plan: registered a new user with a password. then called the update password RPCs, and was able to successfully log in with my new password. the old password no longer worked, and returned an error.

Reviewers: ashoat, jon, bartek

Reviewed By: jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7553