[commtest] Add functions to login/logout existing username

Summary:
The register_user_device (formerly create_device()) always registers a new user.
To manage device list, we need functions to login/logout devices of existing user.
Created functions that call Login and Logout identity RPCs.

Depends on D10420

Test Plan:
Commtest builds, CI passes.
Functions' logic tested in the next diff.

Reviewers: kamil, michal, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10421