Design and implement interface to populate ModelViewer's Visual tree

[sohchatt]

This design allows SceneLoader to supply a Visual tree to the WinUI control much like Lottie-Windows implements IAnimatedVisualSource. As a result, the WinUI ModelViewer control has no dependence on the SceneLoader nuget, and enables anyone to supply their custom scene graphs.

[GeorgeS2019]

This SharpDX based glTF viewer could be a good case study on how to provide a custome schen graphs for C# to make use of the interface to populate Visual tree