FR: Support Cloud Storage

firebase-admin-dotnet does not support Cloud Storage yet.
I'd like to contribute for it.

The simple specifications are as follows:
* Need new class to handle Cloud Storage. Its name is FirebaseAdmin.Cloud.FirebaseStorage as an example.
* FirebaseStorage instance is managed in FirebaseApp class.
* FirebaseStorage instance has Google.Cloud.Storage.V1.StorageClient instance.
* FirebaseStorage instance will return the StorageClient instance associated with the Firebase app.
* Now the Bucket class is returned in other Java/Python SDK. But the Bucket class in .NET is almost data class. It's not neccesary to be returned(It's my opinion).
* When calling FirebaseApp.Delete(), StorageClient.Dispose() is called.

I would like to open a pull request about the above if there are no problems.