[Feature] Implement Apple Sign In authentication method about plugin.firebase

[TobiasBuchholz]

Hi @Kapusch, thank you very much for your feedback and interest in the library!

Unfortunately it's quite hard for me to see the relevent changes in your implementation because of all the code formatting differences.

On the development branch I've added an .editorconfig file that handles the code formatting and can be used to apply formatting rules by running dotnet format src/Plugin.Firebase.sln in the console.

Please checkout that branch, apply your changes and code formatting and create a Pull Request, so I can merge it into the project if everything works well. For more information on how to contribute, take a look at the README.

[Kapusch]

@TobiasBuchholz , I have provided an implementation for Apple Sign In via OAuth credential, it works fine.
You can check the solution in my forked repo : Kapusch@edd947f

If you're fine with that solution, then I'll provide an example in your Samples project.

Just tell me if you would rather prefer having the specific Apple Sign In workflow from Xamarin.Essentials directly in your library, so that it'd be smoother for the developer to simply just call "SignInWithAppleAsync()" with no parameter, as you did for Google & Facebook authentication.

[Kapusch]

Sadly I am currently not much available, but I will keep you informed as soon as I can do it.

[Kapusch]

This will be covered with #37