Implement olm session updater that has encrypting functionality

Summary: This differential implements updater that can use olm session to encrypt data.

Test Plan:

1. Apply those changes to NotificationService.mm: https://gist.github.com/marcinwasowicz/0a43950fb202f03b3c5b92fe17dd5272
2. Build the app
3. Log in
4. Pull from the db cookie id of the currently logged in user.
5. Apply those changes to the send.js in the keyserver: https://gist.github.com/marcinwasowicz/9bc13a92310a9b91a59d73a2557b62a9
6. Add your cookie id.
7. Send the notif.
8. Ensure it is displayed and examine the logs

Reviewers: tomek, atul, bartek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7797