Use microG's Google Play Services library replacement to support Push Notifications for F-Droid users

### Is your feature request related to a problem? Please describe.
F-Droid users cannot get "real" push notifications.

### Describe the solution you'd like
MicroG is developing a replacement library for Play Services client library: ~~https://github.com/microg/android_external_GmsLib~~ https://github.com/microg/GmsCore (**edit:** changed URL)

It's FLOSS so it would AFAIK be allowed to be included in the F-Droid build. And if the F-Droid users do have the original Google Play Services or [microG](https://microg.org/) installed, they could use a real FLOSS app, but also get push notifications. (so stuff like https://github.com/nextcloud/android/issues/922 is not needed)

### Describe alternatives you've considered
Leave it as it is; or remove Google Play Push services entirely. Both not ideal.

### Additional context
It seems to be still in development, but it's an idea, at least.

Überprüfe den vorgeschlagenen Ersatz für microG/GmsCore und die bestehende Integration der Google-Play-Push-Dienste der App und prüfe anschließend, wie der F-Droid-Build mit dieser Abhängigkeit umgeht. Bestätige, ob der Ersatz ausgereift und kompatibel ist; abgeschlossen ist die Aufgabe, wenn F-Droid-Nutzer Push-Benachrichtigungen empfangen können, ohne proprietäre Google Play Services zu benötigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.