Google-cloud-dotnet: Firestore's CollectionGroup queries

[DenisNP]

Is your feature request related to a problem? Please describe.
There is a cool feature in Firestore: collection group queries.
This option is available in almost all supported languages, except several, including C#. Why? It is a killer feature to get away from SQL databases to Firestore.

Describe the solution you'd like
CollectionGroup object in .NET Firestore client with all necessary methods and properties.

Describe alternatives you've considered
Many sequential standard queries. Much more slower and harder to write.

[jskeet]

Collection group queries do exist in C#. Use FirestoreDb.CollectionGroup.

I'll file an internal issue about the lack of snippets for C# for this feature on Monday.

[jskeet]

(Closing now because I don't think there's anything more to do in this repo. If it doesn't work for you, we can reopen.)