[native][web] Add a context provider on native that exposes notifications session creator

Summary:
This makes it more convenient to use this function on both platforms.

Depends on D10662

Test Plan: Check if encrypted notifs work on iOS and web.

Reviewers: varun, marcin, inka, kamil

Reviewed By: marcin, kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10663