Allow to create an instance for a specific Firebase app

## What feature would you like to see?

In most of services (e.g. functions, firestore, auth, storage, etc) we can create an instance for a specific Firebase app.

However, it's not possible in the following services:
* Cloud messaging
* Crashlytics
* Analytics (at least not for mobile)

As far as I know, these services are fixed to the default Firebase app instance created by the SDK.

## How would you use it?

If the feature is implemented, we would be able to create an instance of those services pointing to a specific Firebase instance of our choice.