Allow layer.json and tilemapresource.xml to be omitted in tileset and overlay URLs

For TMS raster overlays:

cesium-native tries the exact user-provided URL first. If that fails, it tries adding `tilemapresource.xml` to the end. But we've had reports this isn't working reliably. It could be its notion of "failed" isn't ideal, or it adds the `tilemapresource.xml` using a relative URL resolution, which doesn't work as expected if the original URL doesn't end in slash. Needs to be investigated.

For 3D Tiles:

CesiumJS and cesium-native both require that the final `/tileset.json` be included. So no change required here.

For quantized-mesh terrain:

CesiumJS does not expect the final `/layer.json` to be included in the URL when constructing a `CesiumTerrainProvider`, but cesium-native requires it. This one is tricky because cesium-native doesn't have a separate type for terrain versus 3D Tiles. So we will have to simply try appending `/layer.json` to every otherwise-unsuccessful URL.