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

[kring]

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.