Refactoring the user management code

[hiranya911]

• Making the user management initialization thread-safe
• Cleaning up the lazy loading code, and the FirebaseAuth.Delete().
• Exposing a new FirebaseAuth.SetCustomUserClaimsAsync() method that accepts a cancellation token as a second argument.
• Introducing a couple of helper functions to Extensions to make it easier to send JSON requests, and tear down managed resources.