[lib] Add function to verify device lists

Summary:
Added function that verifies device lists timestamps and signatures on client side - Whitepaper 6.2.4
Now, we use Identity Service as a source of truth for device list updates.
Signatures are optional at this point, unsigned lists should still be accepted.

Depends on D11901, D11907

Test Plan: Added unit tests in D11930. More detailed plan will be performed when the function is used.

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11929