Create architecture diagram illustrating how 3D Tiles are streamed from Cesium Native

[j9liu]

An extension of CesiumGS/cesium-native#982, but with more focus on the Cesium for Unreal side.

It'd be great to show the integration points that we have with Cesium Native. How have we implemented IAssetAccessor and IPrepareRendererResources, how does ACesium3DTileset interface with cesium-native's Tileset, etc. Basically showing the data flow between Unreal and Native, and an overview of any additional work we do on the Unreal side to process that data (e.g. in CesiumGltfComponent).