Support i3dm tile payloads

[argallegos]

Splitting up this old issue so we can more accurately track progress.

Add support for Instanced 3D Models.

Attempting to load i3dm tilesets results in errors like [warning] [TileContentFactory.cpp:77] No loader registered for tile with content type '' and magic value 'i3dm'

[argallegos]

Here's the cesium-native side of this: CesiumGS/cesium-native#458

[DevElves]

Is there any new progress on this? I3DM is a format type that urgently needs support, I have been looking at this issue for almost two years, and I would appreciate it if you could give priority to implementing this.

[kring]

We haven't observed it to be urgent for most of our users, which is why it hasn't been prioritized. If it's especially urgent for you, @DevElves, we could discuss options for you to fund development of it. We also welcome contributions in the form of pull requests.

[j9liu]

This has been closed by #1371 and is available in the latest Cesium for Unreal release!