Make a widget for passing firebase options

[rrousselGit]

From the example it seems we have to use a custom runFirebaseApp to specify firebase options.

But this goes against the "Everything is a widget" rule Flutter uses.

We most likely can move the custom logic to a Widget.

[rrousselGit]

As a side note, Flutter already has similar behavior.

We get things like ScrollConfiguration, or the whole Localization thing (which can load from http too).