Support standard webpush

[p1gp1g]

Mastodon now support standard webpush: mastodon/mastodon#33528, it will be in mastodon 4.4.0. If we want to migrate clients using FCM we need to support the standard push here too

The content in the p field is enough to decrypt the push notification, rfc is added to know it comes from a standard push notification. Another solution is to simply not set k and s, so if they aren't there, we know it is a standard push. But I think it is better to add a field for that

[ClearlyClaire]

I am not convinced adding a rfc field is useful, but it probably doesn't hurt much either.

[p1gp1g]

By the way, you don't need a gateway for webpush to FCM when using the standard webpush. You can directly send the messages to FCM (google) servers: https://gist.github.com/mar-v-in/2a054e3a4c0a508656549fc7d0aaeb74#webpush