[lib] Add selector to get peers user and device IDs

Summary: Added a selector to conveniently get peers (userID, deviceID) pairs which can be used to e.g. broadcast Olm messages.

Test Plan: Flow. Logic tested later in the stack

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12994