Add KHR_animation_pointer extension support for glTF

[jussimcius]

This is a feature request to add support for KHR_animation_pointer glTF extension. While other 3D rendering engines are starting to support this extension, I did not find any mentions or discussions about this topic by vis.gl community.
Spec documentation: https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_animation_pointer/README.md

[jussimcius]

Also, to add, glb/glTF files, which have such animations, are not correctly parsed by GLTFAnimator and are not working at all.