[web/native] Move notificationsSessionCreator to olmAPI

Summary:
Part of ENG-6657 : Migrate session creation code to shared worker

Simiarly to D11339 this corresponds to notificationSessionCreator from OlmSessionCreatorContext but without the react-based code (as it doesn't work in the shared worker). The code from OlmSessionCreatorContext will be removed in a future diff.

Depends on D11341

Test Plan: Tested in a later diff but in particular -> checked that the web notifications work and were able to be decrypted using this API to create a session.

Reviewers: kamil, tomek, marcin

Reviewed By: kamil, marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11342