Skeletal animation

The skeletal animation of the LayaAir engine includes two major categories: One is to convert third-party skeletons (keel and spine) into the engine's built-in skeleton animation file format. The other is to directly encapsulate Spine's skeletal animation action library. The built-in skeletal animation has better performance, but is limited by the version of the conversion tool and only supports spine versions prior to 3.8.