Add Command Binding sample apps for .NET 7 Command Binding blog post.

[KlausLoeffelmann]

This PRs adds the sample solution for the Command Binding blogpost:

• Contains the Android .NET MAUI App.
• Contains the WinForms App which demos Command Binding in .NET 7.
• Contains the ViewModel, controlling both apps.
• Contains a demo project showing how to Unit Test a simple ViewModel.