Clean up InitializeNavigationAsync extensions

[dansiegel]

Currently the Navigation & Toolkit Navigation each provide an extension InitializeNavigationAsync on the Window. This should be cleaned up:

Remove Toolkit Navigation overload

Introduce an interface IInitializeViewHost that will allow the Toolkit to provide an implementation that can be resolved by the Host to configure the Loading View with the Startup Task.

// In Extensions Navigaiton
public interface IInitializeViewHost
{
    void InitializeViewHost(FrameworkElement element, Task startupTask);
}

// In Toolkit Navigation
internal class ToolkitInitializeViewHost : IInitializeViewHost
{
	public void InitializeViewHost(FrameworkElement element, Task startupTask)
	{
		var loading = new LoadingTask(startupTask, element);
		if (element is LoadingView lv)
		{
			lv.Source = loading;
		}
	}
}

Add ShowAsync Extension for IApplicationBuilder

To better support the IApplicationBuilder add a reference to Extensions.Hosting.WinUI and add ShowAsync method to Extensions Navigation. Together with the IInitializeViewHost simplification this would allow it to work across both libraries. This will also clean up the Extensions Template.