[commtest] Generate random client keys by default

Summary:
Depends on D12340.

First step to get rid of hardcoded device IDs
Made login and register functions use ClientPublicKeys default implementation from D12340, if no explicit keys provided.

Test Plan: Commtest

Reviewers: kamil, will, varun, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12341