Add OTP Metadata Encoding for Safe Backups

[gaetanomatonti]

Description

When using a device for multi-factor authentication through OTPs, metadata should be stored and backed up (on a cloud solution presumably) so that users won't lose access to the authentication when deleting the app or switching device.
A good approach would be to encode the metadata (updated with the correct counter for counter-based OTPs), encrypt it so that only the app is able to decrypt it (maybe with CloudKit encrypted fields) and providing it to the client for backup.