Add support for Mocking

As far as I can tell, all IFirebaseServices are sealed and have internal constructors.

This presents us with the challenge of how we can test code, which uses the FirebaseAdminSdk, which contains very critical operations.
I ask you to re-evaluate this design decision, as mocking is required to achieve a high test coverage of our own code, which interfaces with FirebaseAdmin SDK directly.

In my project, I was able to identify two critical firebase services, which lack mockability:
- FirebaseAdmin.Messaging.FirebaseMessaging
- FirebaseAdmin.Auth.FirebaseAuth

My environment:
- .Net Core 3.1
- NUnit 3.12.0
- Moq 4.14.1