Add option to preload a tileset or section of a tileset

This was suggested by a [user on the forum](https://community.cesium.com/t/possible-to-pre-load-large-parts-of-a-dataset-to-avoid-obvious-popping-lod-visual-glitches/14122).

There are times when users may need a tileset to be fully loaded before beginning play, or may want to delay at any point until a tileset is fully loaded. It would be a great feature if users could designate either a tileset to preload, or draw out a section of a tileset to preload. In practice, I imagine this would involve waiting until the tileset is fully loaded before drawing anything to the screen, presumably pausing physics and other simulation until then. This feature could be set to run when the user hits start, or triggered manually via blueprint or code.

Of course, depending on the detail of a given tileset and a user's machine, this setting could get out of hand fast in terms of performance and memory management. It also wouldn't be desirable in many situations, but is specifically helpful when users need a particular section of a level to be reliably loaded.

Maybe not top priority, but definitely a feature worth further discussion.