Voxel API should conform to Cesium3DTileset interface

[ggetz]

Right now Cesium3DTilesVoxelProvider and other voxel classes are distinct from how other 3D tilesets are used in CesiumJS:

const provider = await Cesium.Cesium3DTilesVoxelProvider.fromUrl(
  "..."
);
const voxelPrimitive = viewer.scene.primitives.add(
  new Cesium.VoxelPrimitive({
    provider: provider,
  })
);

The goal is eventually to bring voxels into 3D Tiles as an extension. As such, we want to treat it in the API like like other 3D Tilesets, and be able to operate using familiar paradigms. Furthermore, the properties and functions associated with other 3D Tilesets should be available for voxel tilesets too, particularly:

• the loading events, like allTilesLoaded, initialTilesLoaded, loadProgress, tileFailed, tileLoad, tileLoad, tilesLoaded, tileVisible, tileUnload.
• statistics (for use in other parts of the API)

Where possible, we should also make sure the documentation is in good shape, and include inline examples where we can.

CC #11018
CC #11008