[firebase_messaging] check permissions

[wwwdata]

Description

This change introduces a method call for iOS to check whether the user has previously accepted or declined the push notification setting. This is useful to check it later in the app and then remind the user to enable the push notifications manually in the control center, because the initial request permissions dialog is only displayed once. Or if the user changed the permissions in the Settings, and switches back to the app, the onIosSettingsRegistered event is also not fired.

Related Issues

I stuck to the old API, if flutter/plugins#1009 is merged in, this should also be updated to use the new UNUserNotificationCenter

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• 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.

[genert]

This PR has conflicts against master branch, please resolve them first.

[genert]

Ping @collinjackson @kroikie

[kroikie]

@wwwdata Many thanks for this PR. We try to keep the API as close to the native SDKs as possible. I think this is indeed a useful feature but I don't think it is something that should be part of the FCM plugin. I think the Flutter framework would be a better fit for a system specific API like this one.