Add support for loading baseline glTF 2.1 models

[jjhembd]

Feature

As part of #13528, we will need to update our glTF loading pipeline to support models conforming to the glTF 2.1 spec.

This issue will be used to track the work to load a baseline glTF without extensions--so it will not include 3D Tiles 2.0 work.

A preliminary list of the tasks involved:

• Locate or generate test datasets
• Support glTF 2.1 models gltf-pipeline#681 (related: gltf-pipeline as an npm module? #9614)
• Support changes to core, such as external assets, bounding volumes, unified file references, unique IDs
• Handle extensions promoted to core

Most (all?) of the above will have to be implemented before we can "turn on" glTF 2.1 models.

Other new 2.1 functionality is specified via extensions. These can be implemented more incrementally, with separate issues/PRs for each extension.

AI acknowledgment

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• (If the above is checked) I have reviewed the AI-generated content before submitting.

[jjhembd]

@javagl are you aware of any existing test datasets?

[javagl]

I'm not aware of existing glTF 2.1 assets. And I doubt that there are many of these, given that the spec development is pretty much in flux right now. There probably are only the "snippets" in the explainers and specs. Maybe some baseline assets could be assembled from that with reasonable effort...

(I plan to support glTF 2.1 in JglTF. But - even though I recently had an unexpectedly high amount of "spare time" - there's no timeline for that)