Firebase Messaging to work with a non-default FIRApp

### [READ] Guidelines

For large or ambiguous requests, such as significant breaking changes or use
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starting a
[Pitch](https://github.com/firebase/firebase-ios-sdk/discussions/categories/pitches)
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flush out.

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* is either immediately obvious (i.e. "Add Sign in with Apple support"), or
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includes an API proposal that would make the use case possible. The proposed API
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### Feature proposal

* Firebase Component: Messaging

Firebase supports creating multiple instances of its core component - FIRApp. Currently, Firebase Messaging on iOS works with the default FIRApp only. Thus there is no way to make an iOS app receive push notifications from a different source than its main Firebase project. At the same time, Firebase Messaging supports working with non-default FIRApp instances on Android.

When will Firebase Messaging support non-default FIRApp instances on iOS?