[native] generateAndGetNotificationsPrekey

Summary:
We'll need to send the notif prekey/prekey signature to identity service. Exposing a function that lets us generate and get a notif prekey.

Depends on D9076

Test Plan: tested in subsequent diff

Reviewers: atul, marcin

Reviewed By: marcin

Subscribers: bartek, ashoat, tomek

Differential Revision: https://phab.comm.dev/D9083