[services][identity] helper function to finish PAKE login

Summary:
helper function completes PAKE login and returns an access token or an error depending on the outcome

depends on D4350

Test Plan: tested with method that calls this helper

Reviewers: palys-swm, jimpo, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4351