[firebase_core] Allow one configuration file for all plattforms

[IchordeDionysos]

Is your feature request related to a problem? Please describe.
In the Roadmap #2582 is the effort described to "Reduce the installation steps".

Adding the config file for every platform is cumbersome, adding just one file for all platforms would be great.

Describe the solution you'd like
I would settle on some kind of format (maybe JSON) and folder name for a Flutter config file.

This config file should be added to the application by the developer and firebase_core would try to read that file first to configure any Firebase app using these methods Android, iOS, web.

Additional context
For the priority, I would first look at the Firebase config file and then try to use the platform-specific config files.

JSON as a format could be helpful as a format as one could for now just copy the web config.

@FirebaseExtended/invertase @Salakar Could be relevant for the core rework invertase/flutterfire#7

[Salakar]

@IchordeDionysos appreciate you taking the time to write this out, unfortunately this isn't something we're going to be looking at, at least not as part of #2582 - there's some complications as each platform has their own specific keys and ways of initializing these credentials, plus the way to retrieve each of these credentials from the Firebase Console will still be the same and produce the same output, so there'd have to be something that wraps around these.

Having said that though; we are taking steps to make it easier for you to figure out how to integrate the credentials, e.g. we've got a new docs site in the works with specific platform integration sections: https://firebase.flutter.dev/docs/overview#platform-setup - and we're also taking steps to reduce manual native code changes that are normally required to integrate.