Open sources push notification

### Is your feature request related to a problem? Please describe.
I'm using a "de-googled" android phone and I'm using Nextcloud as a open-source and independent option for "like-google" and "like-apple" services.
In the android app, under notification I got an error message "Push notifications are disabled because they need Google Play Services".

### Describe the solution you'd like
I would like to have a more free app that can work out his push notification without needing Google Play Services help. In more recent years many many applications started to develop their independent push services, so I hope it is "easy" to implement it.

Thank you.

(Easy step to reproduce the "problem": open Nextcloud android app with a phone without play services or MicroG and open "notification" tab).