Adopt `rendererOptions` pattern from `RasterOverlayOptions` for `TilesetOptions`, too

[kring]

In #471, we added a new rendererOptions field to RasterOverlayOptions. This allows a renderer (e.g. Unreal Engine / Cesium for Unreal) to pass data to an overlay that is then passed back to the renderer via IPrepareRendererResources::prepareRasterInLoadThread. This is important because many objects in both cesium-native and the renderer itself are unsafe to access from this background thread.

We should add a similar field to TilesetOptions and pass it to IPrepareRendererResources::prepareInLoadThread, for the same reason. See discussion here: https://github.com/CesiumGS/cesium-unreal/pull/698#discussion_r844726486