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

Summary: fetch registration data from database, generate challenge for user, return challenge or error

Test Plan: tested with method that calls this helper function

Reviewers: palys-swm, jimpo, karol-bisztyga

Reviewed By: palys-swm, karol-bisztyga

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4350