[native] helper to handle registration response

Summary:
I'm switching from looping through responses from the Identity service to having separate response handlers for each response type.

Depends on D4854

Test Plan: cargo build. this helper will be tested in a diff later in the stack that uses it

Reviewers: tomek, karol, atul

Reviewed By: tomek

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4855