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.

Issue number and link

Resolves #12368.

Related: #12297

Testing plan

TODO

Remaining work

The helpers for VoxelTraversal.prototype.update have been reworked to separate keyframe node selection (the true purpose of "traversal") from data loading and processing.

Next:

• Update sample datasets to the new format, verify specs
• Move 3D Tileset examples to a separate Sandcastle
• Update Cesium3DTilesVoxelProvider.prototype.requestData to return a GltfLoader, which will then be attached to the KeyframeNode in the requestData function in VoxelTraversal. Then updateKeyframeNodes can call gltfLoader.process, perhaps even via a keyframeNode.process function? Or should KeyframeNode be pointing to a child class similar to Cesium3DTileContent?
• Decouple megatexture sorting etc. from keyframeNode processing. Perhaps we should have a megatexture.sort functionality? And make megatexture.add handle the ejection of lower priority nodes when full?
• Move voxelTraversal._highPriorityKeyframeNodes to VoxelPrimitive, and update the nodes from VoxelPrimitive.prototype.update for better alignment with Cesium3DTileset.prototype.update

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]

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