Design and implement interface to populate ModelViewer's Visual tree

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.

Start by reading the ModelViewer control and SceneLoader integration, then compare the requested interface with how Lottie-Windows implements IAnimatedVisualSource. The work is complete when ModelViewer can receive a Visual tree from SceneLoader or another provider without depending on the SceneLoader NuGet package, while supporting custom scene graphs.

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