- This change adds a project file to build the Mixed Reality Authentication shared code to ensure it stays healthy.
  - It took a while to figure out how to add a project that wasn't going to be packaged and shipped, but could also access shared code and what not. You'll see me defining the `AzureCoreSharedSources` property manually because that property is only set for official client libraries which have certain criteria like starting with "Azure.".

  - This change adds a new Share.Azure.MixedReality.Authentication.Tests project to test the shared code. I did find 1 bug that presented when compiling with .NET 5.0 due to better nullable annotations. I had to pull the shared code in as source because I couldn't get `InternalsVisisbleTo` to work in this repo.