[commtest] Add default impl for ClientPublicKeys

Summary:
Some improvement to the ClientPublicKeys type:

• Added new() function that creates keys with given device ID
• Added default() which uses random device ID

Test Plan: Commtest. This is used in other diffs

Reviewers: kamil, will, varun, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12340