[native] helper functions for PAKE login workflow

Summary:
Helper functions that handle the PAKE login workflow.

Admittedly, there's some code from the RegisterUser workflow repeated here. Because the gRPC message is slightly different (RegistrationResponse vs LoginResponse), it's hard to handle both response types in a single function without a lot of match statements and nesting. I'll create a task to make this more DRY.

Depends on D4905

Test Plan: cargo build, tested later in login_user method

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D4906