[firebase_messaging] Separate onLaunch to a future to be able to retrieve launch message in a synchronous way

[jherencia]

Separate onLaunch to an specific method to be able to retrieve launch message in a synchronous way

Description

When an application is closed an a user opens it by clicking a push notification we can retrieve the message in a synchronous way but the current architecture expects this to be asynchronous which may conflict with other processes run in parallel. This behaviour forces programmers to synchronize those processes and with the added problem that on of the async calls (onLaunch) is not called when the user opens the app in a normal way, so it is needed to add timeouts to limit the time we wait the callback.

Android and iOS let programmers access how the app has been opened in a synchronous way for Dynamic Links, Push notifications, etc. So this PR (and #1687) fixes this and handles this two situations in a consistent way.

Checklist

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• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
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• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[jherencia]

Migrated to firebase/flutterfire#1481