Add support for UnifiedPush

[BDGreyson]

This might also be an alternative or addition for #104.

Maybe at some point "UnifiedPush" could be used as an alternative for Firebase Cloud Messaging for de-googled phones, or just people preferring it:

UnifiedPush: https://unifiedpush.org/
UnifiedPush Github: https://github.com/UnifiedPush

This could work for de-googled phones but save battery compared to time-based polling.

If needed, they also have a fallback implementation for phones that have FCM, but also preferred UnifiedPush: https://unifiedpush.org/developers/embedded_fcm/

As a non-programmer I have no estimate on how much work this might be, but considering that you do consider time-based polling, maybe this is also a viable solution.

Thank you either way!

[lukashaertel]

@Fenrikur we can assess this solution and maybe include something for next EF

[lukashaertel]

From what I can tell that's no iOS compatibility with this library.