Add MAUI Sample

[TheCodeTraveler]

This Pull Request adds a .NET MAUI sample app

Fixes #49

I've also updated the Branch Protection rules for master to include both Windows + Mac, because I updated the CI/CD pipeline so that it runs on both operating systems. This is necessary because the MAUI iOS + MacCatalyst app won't compile on Windows, and the MAUI Windows app won't compile on Mac.

This PR also removes the CI Build Steps for the Xamarin.Forms solution because Xamarin is now out-of-support and the tooling is no longer available on the Azure Pipelines hosted agents.