Add support for google-independent open source unified push

[jensMF]

I tried to add support for firebase push to my server and a self build copy of traccar manager, but it did not work. I hope with unified push it might be possible to use the app without rebuilding.

I also like the idea to be google-independent. It would add push support for devices whitout google push, too.

It would also be one step to a completely open app that could be added to f-droid (without the lack of push notification).

Describe the solution you'd like
I would be happy, if traccar manager would implement unified push (more info and help for implementation via https://unifiedpush.org/)

Describe alternatives you've considered
For the purpose of receiving push notifications from custom servers without a custom build of the app, it might be possible to have a setting to change the firebase push token. But I would prefer a google free option.

Additional context
It might be neccessary to implement unified push to the server code too.