Implement building encrypted notifications on the client

Summary: This differential implements full notifications structure generation on the client.

Test Plan:

1. Apply this patch
2. Send message from native device
3. Examin the console output.

Shortly I will make this test plan more extensive to test all notif types.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12477