Save encrypted payload hash in MariaDB keyserver for iOS notifs.

Summary: This differential saves encrypted payload hash for each encrypted iOS notification in MariaDB. Combined with the parent differential this differential enables us to check if there is a match between the hash of notification delivered to iOS device and notification sent from the keyserver.

Test Plan:

1. Build iOS app.
2. Send notification.
3. Check in MariaDB that there is an entry with delivery that contains non-empty array of sha 256 hashes.

Reviewers: kamil, varun, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9179