Support voxel tilesets containing glTF tiles using the EXT_primitive_voxels extension

[jjhembd]

Description

This PR updates VoxelPrimitive and related classes to support tilesets using the proposed update to EXT_primitive_voxels.

Along the way, the internal APIs were cleaned up and made more consistent with Cesium3DTileset, to prepare for work on #12297.

Key code changes include:

• The VoxelProvider interface always returns a VoxelContent, rather than an array of metadata. This affects not just Cesium3DTilesVoxelProvider, but procedural data as well--see the Sandcastles for an example.
• VoxelContent can now be constructed before the data is ready, via the asynchronous VoxelContent.fromGltf method, and updated updated via VoxelContent.prototype.update. This makes it more similar to Cesium3DTileContent. For synchronous construction (e.g. with procedural data), use VoxelContent.fromMetadataArray.
• VoxelTraversal was refactored to use smaller functions that are structured more like Cesium3DTilesetTraversal. For example, loadAndUnload was broken into selectKeyframeNodes (similar to Cesium3DTilesetTraversal.selectTiles) and updateTiles. The updateTiles loop is where we call VoxelContent.update to process glTFs.

Test datasets were also updated to the new format. Note that the tilesets in Apps/SampleData/Cesium3DTiles/Voxel/ are a copy of Specs/Data/Cesium3DTiles/Voxel.

Issue number and link

Resolves #12368.

Related: #12297

Testing plan

Load the three Voxel-related Sandcastles and verify all datasets load and functionalities are unchanged.

• http://localhost:8080/Apps/Sandcastle/index.html?src=Voxels.html
• http://localhost:8080/Apps/Sandcastle/index.html?src=Voxels%20in%203D%20Tiles.html
• http://localhost:8080/Apps/Sandcastle/index.html?src=Voxel%20Picking.html

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[j9liu]

Could this change to retrieve the property names from the glTF's property attribute? The attribute for a temperature property might be named _TEMPERATURE, which will mess with the find function below.

[j9liu]

@jjhembd do you mind also accounting for CesiumGS/3d-tiles#780? (still working on test data, but I'll get it to you ASAP)

[j9liu]

Oops forgot to mention, could we add EXT_implicit_cylinder_region and EXT_implicit_ellipsoid_region to this list? They're not required for our implementation but they'll be included in generated cylinder / ellipsoid voxels

https://github.com/CesiumGS/glTF/tree/ext-primitive-voxels-revisions/extensions/2.0/Vendor/EXT_implicit_cylinder_region
https://github.com/CesiumGS/glTF/tree/ext-primitive-voxels-revisions/extensions/2.0/Vendor/EXT_implicit_ellipsoid_region