[services][identity] helper function to start PAKE registration

Summary:
checks the state of the stream (that the number of messages received matches expectation), starts PAKE registration, updates state, and returns a RegistrationResponse

Depends on D4352

Test Plan: Tested along with the following two diffs by calling the register_user RPC

Reviewers: jimpo, palys-swm, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4354