Add interfaces for FirebaseMessaging to use DI and mocking in the real projects

[Toloymak]

Add interfaces for FirebaseMessaging to use DI and mocking in the real projects Move XML comments to the interface to keep it in one place having single implementation and keep the chance to keep a full description using an interface by consumers

https://github.com/firebase/firebase-admin-dotnet/issues/196 https://github.com/firebase/firebase-admin-dotnet/issues/139

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