Add payload encryption

[emersion]

This could be done in a Web Push compatible way by using Content-Encryption, see https://www.rfc-editor.org/rfc/rfc8291.html.

The party generating the payload needs to encrypt, and client libraries need to decrypt. The rest of the infrastructure can just pass-through the encrypted data.

Ref ircv3/ircv3-specifications#471, which has some WIP patches for this. See https://git.sr.ht/~emersion/PushGardenTestApp for a decryption example on Android.

[p1gp1g]

Encryption has to be handled by the application and is out of scope of UnifiedPush. For instance, Mastodon uses web push, so notifications are encrypted.

Actually, we will do (or wait for) a web push over unifiedpush library (UnifiedPush/wishlist#15)