Support 3D Tiles Voxels

### Feature

We're slowly rolling support for [`3DTILES_content_voxels`](https://github.com/CesiumGS/3d-tiles/tree/cesium-native/extensions/3DTILES_content_voxels) into Cesium for Unreal; there is interest to bring this to Unity, too.

One challenge is the difference in how we will handle creation of materials/shaders for the voxel dataset. We still lack support for regular metadata styling (#200), so there's nothing existing to build off of here. So we'll need to find the best entry points that would allow users to write custom shaders to style the voxels are they're raymarched.

The issue names no Unity files, tests, or concrete entry points. Start by reviewing the 3DTILES_content_voxels specification and the existing Cesium for Unreal voxel support, then identify Unity material and shader entry points for raymarched voxels. Done should include voxel support and a defined way for users to provide custom styling shaders.

Written by the indexing model from the issue text.