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]

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

[ggetz]

Thanks @jjhembd!

Tests and examples are working well! I had a few comments around the API, but otherwise the code is looking good too.

Let me know once you get a chance to address those and @j9liu's comments.

[jjhembd]

Thanks for the feedback @j9liu and @ggetz! I think I addressed everything.

[ggetz]

Thanks @jjhembd! Could you please merge in main?

[ggetz]

This is looking great @jjhembd! Just a few (hopefully) small comments, and then this should be good to go.

[jjhembd]

Thanks @ggetz, I think I incorporated all those comments. This should be ready for another look.