KHR_animation_pointer support

[Igor-dvr]

The base glTF 2.0 specification supports animating only node transformation or morph target weights. With this extension, an animation can target any mutable property in a glTF asset, such as material factors or camera field of view.

https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_animation_pointer/README.md

Godot implementation godotengine/godot#94165

[Igor-dvr]

Gltf crate animation pointer PRs that should simplify adding support in Bevy:

• Add extension KHR_animation_pointer gltf-rs/gltf#468
• Rework for version 2.0 gltf-rs/gltf#422