Set maximumTileCount in Cesium3DTilesVoxelProvider

[lilleyse]

VoxelProvider has a maximumTileCount property that is used to preallocate various data textures of a VoxelPrimitive. This property used to exist in Cesium3DTilesVoxelProvider but was removed prior to #10253 being merged because it was not general purpose enough.

The approach in this PR is to look for tileset metadata that uses the TILESET_TILE_COUNT semantic. In 3D Tiles 1.1 this would look like:

  "schema": {
    "id": "voxel",
    "classes": {
      "tileset": {
        "properties": {
          "tileCount": {
            "type": "SCALAR",
            "componentType": "UINT64",
            "semantic": "TILESET_TILE_COUNT"
          }
        }
      }
    }
  },
  "metadata": {
    "class": "tileset",
    "properties": {
      "tileCount": 401
    }
  }

The reason this PR is important is because without maximumTileCount a VoxelPrimitive will allocate a megatexture and node texture that far exceed the actual memory usage required by smaller tilesets.

See the corresponding 3D Tiles PR: CesiumGS/3d-tiles#728

[cesium-concierge]

Thanks for the pull request @lilleyse!

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[jjhembd]

Looks good!