[lib] Add hook to broadcast Olm-encrypted messages

Summary:
Address feedback from https://phab.comm.dev/D12944?id=43066#inline-75283
Extracted sending Olm messages (and optional session creation) to a hook.
It is used in a few places, which are updated in subsequent diffs

Test Plan: Flow. Logic tested in later diffs

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12995