[services][identity] pake registration finish helper

Summary:
this helper function does some input validation (makes sure the right number of messages have been received, user ID has been provided), completes PAKE registration, and uploads the user's registration data to DynamoDB

Depends on D4354

Test Plan: tested along with subsequent diff by calling the register_user RPC

Reviewers: palys-swm, jimpo, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4359