This commit adds support to send encrypted push notifications
to devices registered to receive encrypted notifications.
URL: `POST /api/v1/remotes/push/e2ee/notify`
payload: `realm_uuid` and `device_id_to_encrypted_data`
The POST request needs to be authenticated with the server’s
API key.
Note: For Zulip Cloud, a background fact about the push bouncer is
that it runs on the same server and database as the main application;
it’s not a separate service. So, as an optimization we directly call
'send_e2ee_push_notifications' function and skip the HTTP request.
7e1afa0e8a