[native] register_user client method

Summary:
This method handles the bidi streaming interaction with the Identity service for registering a user. Some of the logic has been moved to subroutines that can be found earlier in the stack. I tested this method by creating a main function and calling it, and successfully receiving an access token. I tested the error cases by modifying the Identity service (running locally) to simulate various siutations (wrong response returned, server hangup, invalid response bytes) and checked that they were handled correctly on the client side by inspecting the logs.

Depends on D4857

Test Plan: See the summary above

Reviewers: tomek, karol, atul

Reviewed By: tomek, karol

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4802