[Keyserver/rust] Add device login and register

Summary:
Rust portion of keyserver login/register.
Largely adapted from integration tests.

Depends on D8365 D8052

Test Plan: Tested in later diff

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8366