[native] helper to handle credential response from Identity service

Summary:
This helper function checks that the message is what we expect, and then completes PAKE login and returns a message to the Identity service.

Depends on D4855

Test Plan: cargo build. this will be tested later by the function that uses the helper

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4856