Support for KHR_animation_pointer

[bobbens]

KHR_animation_pointer was ratified and merged earlier this year and would significantly boost animation potential by allowing animation things like textures and pretty much everything. I think it would be very beneficial to have support for it.

[crazyjackel]

I would like to second this, with some thoughts regarding implementation details:

1. Currently GLTF-RS is not in spec due to the Index::Node on Animation Channel Target being required counter to 5.7

2. Target Path needs to be updated to include "pointer" as a valid value; This is difficult to control via feature flag, there is a good argument to handle this similarly to how Image::MimeType is handled compared to spec by just using raw strings with comments on the allowed values. Though, we can manage it with a feature flag by making Target Path non-exhaustive, which is my preferred solution.

3. Extensions need to be populated with the Pointer provided. The precise typing on this is difficult as it references the asset, but KHR_animation_pointer is explicit that it must be mutable in the glTF 2.0 Asset Object Model Specification, which means we can fully enumerate the precise references. I would recommend making a GLTFMutablePointer enumeration that can be serialized and deserialized from the provided pointer templates. This Enum would also need to be non-exhaustive due to extension variants that are also defined in Asset Object Specification.

I think the first step is reconciling # 1 because 'gltf-rs' is out of specification ATM... let me know if you would like me to write up a PR for this implementation.

(Edit: step 1 was done in #439 and PR #445, instead of being optional, it is instead set to max value with an is_empty being used. I missed this because this version has not been released.)