Make default materials accessible to users

If you create a tileset and click on the "Opaque Material" slot, it won't show the Cesium default materials included in the Cesium for Unity Package. It seems like that window only brings up materials available in the `Assets` folder. We should try to find a way to make those materials accessible, whether if its from the package itself, or by importing the materials (and other user-accessible resources) into a folder under `Assets`.

Start at the Unity tileset inspector's "Opaque Material" slot and trace how its material picker discovers assets. Check how the Cesium for Unity Package exposes its default materials, then make those resources selectable from the slot; done means the default Cesium materials appear for users without requiring an existing Assets-folder copy.

Written by the indexing model from the issue text.