[native] another helper for PAKE registration on client side

Summary:
This helper kicks off PAKE registration on the client side. The register_user method has a lot of logic, so I'm moving the PAKE parts to subroutines.

Depends on D4813

Test Plan: tested in subsequent diff

Reviewers: tomek, karol

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4814