[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