[FirebaseMessaging] Support for multiple Firebase apps

[vmalyi]

When adapting our app to multiple environments (production, test, development, etc.), I've stumbled upon FirebaseMessaging's private instanceFor constructor which docstring states that FirebaseMessaging doesn't support multiple Firebase apps.

This is very unfortunate because it prevents us from using the phone verification on iOS in all additional Firebase environments except the default one. Additionally, it causes FirebaseMessaging not to work in our non-default Firebase apps.

I also noticed other folks facing similar issues because of the lack of the multiple apps support like #5844.

I'm curious what are the plans for FirebaseMessaging to support multiple apps the way e.g. FirebaseAuth, FirebaseFirestore are already doing?

Thank you in advance!