[services][identity] PAKE workflow enum

Summary: This enum lets us determine if a PAKE login request is part of a registration or login workflow, so that we can send back the appropriate response struct

Test Plan: cargo build worked

Reviewers: jimpo, palys-swm, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4360