[Feature]: push notifications not based on google fcm

[palight]

Problem / Motivation

As most of people that host their mailserver do it in purpose to put out gafam of their lives, it will be good that push notifications are not build on Google FCM for the android build (or to offer alternative choices).

Proposed Solution

As this is done for mercurygram (telegram fork) or molly (signal fork) an option to enable push notifications with unified push server would be nice.
Another alternative would be to use the Event Source feature of Stalwart server.

Alternatives Considered

Stick with Google FCM system 😣

Feature Area

Notifications

Mockups / Examples

https://github.com/Mercurygram/Mercurygram#unifiedpush-webpush-gateway
https://github.com/mollyim/mollyim-android#unifiedpush

Additional Context

No response

[rathlinus]

I already had unified push working, but noone used it, but I can add it back

[palight]

It would be wonderful !