PR support KHR_animation_pointer

The Sample Models repo had a [PR ](https://github.com/KhronosGroup/glTF-Sample-Models/pull/353) for KHR_animation_pointer. The PR was created by [hybridherbst](https://github.com/hybridherbst) and included a model that tested the extension. This issue is to keep the connection in the new repo. The PR was from https://github.com/prefrontalcortex/glTF-Sample-Models/tree/KHR_animation_pointer

Start by reviewing the referenced glTF-Sample-Models pull request and the KHR_animation_pointer branch linked in the issue. The issue does not name files, tests, or acceptance criteria, so confirm how the sample should be represented in glTF-Sample-Assets and what maintainers consider complete.

Written by the indexing model from the issue text.