Allow to create an instance for a specific Firebase app

[mauriciotogneri]

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.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.