Asset manager needs addAnimationTask to load animation only files

[PatrickRyanMS]

When working with skinned meshes and animations, it is common to have the final skinned mesh asset separated from its animation files which will be loaded individually. When using asset manager, we have tasks for meshes, but no task to load a file that has been saved with animations only. These files could be exports from Blender where the mesh is hidden or using the Babylon exporter and enabling "animations only export."

This should work similar to SceneLoader.ImportAnimations, however I believe that ImportAnimations will automatically assign the animations to a skeleton in the scene and start them playing. If we are loading multiple animations with Asset Manager, I think we can expect the user to want to manage where those animations are applied, especially if there are multiple characters in the scene.