[native] another helper for PAKE login on client side

Summary: This helper function starts PAKE login on the client side. It handles some error mapping and splits out the request to send to the Identity service and the ClientLogin object we keep to finish login later.

Test Plan: tested two diffs later

Reviewers: tomek, karol

Reviewed By: tomek

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4813