chore(firebase_messaging): migrate to depend on new firebase_core plugin

[Salakar]

Description

• Depend on firebase_core pre-release versions and migrate plugin to use firebase_core native SDK versioning features;
  • Firebase iOS SDK versions are now locked to use the same version defined in firebase_core.
  • Firebase Android SDK versions are now using the Firebase Bill of Materials (BoM) to specify individual SDK versions. BoM version is also sourced from firebase_core.
• Allow iOS to be imported as a module.

Related Issues

N/A

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• 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 ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• 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.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

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.