Add basic support for voxel rendering and styling

[j9liu]

Description

Depends on CesiumGS/cesium-native#1188.
Depends on #1694, so merge that first.

This PR adds support for loading and rendering tilesets with 3DTILES_content_voxels, parsing glTFs with EXT_primitive_voxels payloads.

Data courtesy of Swisstopo

This is a meaty set of changes, so I'll add a written walkthrough of the changes in a follow-up comment.

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
  - [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
• I have updated the documentation as necessary.

Remaining Tasks

• Improve memory / lifetime management
• Revisit voxel traversal mechanics. -- See changes in Add support for ellipsoid-based voxels #1711
  • I'm not happy with the way voxels are currently selected for rendering in VoxelResources. It's a weird compromise between CesiumJS's VoxelTraversal and the cesium-native traversal, and it falls apart for larger datasets.

Testing plan

Reach out to me offline for the pictured test data.

• Load in a voxel tileset (same as any other Cesium3DTileset).
• Use UCesiumVoxelMetadataComponent to gather its properties.
• Test out applying custom shaders to the data.

Compatibility checks:

Unreal Version	DX 11/12	Vulkan	Metal	Editor	Built
5.4	✅	✅		✅
5.5
5.6

[j9liu]

I could keep picking apart at this, but I think this would benefit more from a fresh pair of eyes. I'm keeping this as a draft but I would appreciate any and all reviews 🙏

[j9liu]

Turns out that UE 5.4 has drastically improved the UX of Custom HLSL nodes in materials!

It's now possible to define extra #include paths in for the shader, so I don't need to use one uber shader anymore. I've reorganized the code so that everything is separated into meaningful pieces. Hopefully this makes things easier to review, too ^^