[all plugins] All plugins should have a static instance member, and a static instanceFor() method if arguments are allowed

[collinjackson]

This is a breaking change that we'd like to land as part of our push towards stabilizing the Flutterfire API and authoring integration tests that work across web and mobile (see #40).

Currently the Flutterfire plugins are inconsistent about how you get an instance of the plugin. Some, like firebase_analytics, offer only an anonymous constructor: FirebaseAnalytics(). Others have a static final instance member. Some, like cloud_firestore, have both. The anonymous constructor sometimes takes additional arguments, as in the case of firebase_storage.

The following change is proposed:

• All plugins (except core) should have a FirebaseXXX.instance member that uses the default Firebase app.
• If the plugin offers optional named constructor arguments such as multiple app support, a FirebaseXXX.instanceFor() factory constructor should be provided.
• The method should return the same instance when passed identical arguments.
• Anonymous constructors should be deprecated and removed in a subsequent release.

[Salakar]

Hey, this is being addressed as part of our on-going work for #2582, with Core & Firestore already being standardized with other plugins to follow in the coming weeks.