Add UnifiedPush option for notifications (e.g. via NextPush)

[oliob]

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Feature request

Which Nextcloud Version are you currently using: (see administration page)
30.0.6

Is your feature request related to a problem? Please describe.
Using a de-googled cellphone i want to get notifications from Talk. https://unifiedpush.org/ seams a potential solution since i can run a notification service within my NC server instance and subscribe my phone to it via the app: NextPush.
The issue is however, NC Talk (android) app relies on FCM for push notifications and if i use it out of F-droid it is not compiled with any notification possibility and i cant register the app within NextPush on the cellphone to receive notifications.

Describe the solution you'd like

In an ideal world i would like to get Unifiedpush notfifications sent via FCM if client runs on stock android and via e.g. NextPush if it runs on a de-googled phone.

Describe alternatives you've considered
Using NextcloudServices is ok but it is not PUSH! its POLL!!
And it would be much more versatile having Unified Push implemented directly into Talk and NC

Additional context
Add any other context or screenshots about the feature request here.

[nickvergessen]

Talk itself does not do notifications. It's handled by the notifications app and there the request is tracked and WIP in nextcloud/notifications#2027