Implement peer notifications session creation as outbound on web and add notifs outbound session creator to OlmAPI

Summary: This differential implements outbound notifications session creation on web. It also adds this functionality to OlmAPI so that code in lib can create notifs sessions

Test Plan: Tested in D12673

Reviewers: bartek, tomek, inka, kamil

Reviewed By: tomek, kamil

Subscribers: kamil, ashoat

Differential Revision: https://phab.comm.dev/D12669