Allow customization of tile GameObjects, perhaps using a prefab

Users often want to be able to customize the game objects and components that are created for each loaded `Tile` for various reasons. For example, this recent community forum thread:
https://community.cesium.com/t/how-to-disable-shadow-casting-receive-only-on-tileset/40673

One good approach for doing that is to subscribe to the `OnTileGameObjectCreated` and make whatever modifications are necessary there. But it would be useful, and perhaps a bit more efficient, to let the user provide a template for the creation of the game objects in the first place, rather than modifying them after creation. Unity's prefab system could be a natural way to accomplish this.