Implement Exponential Height Fog

[jjspace]

When working on https://github.com/CesiumGS/cesium/issues/12126 I discovered that Unreal has a feature for it's atmosphere/fog to change the density based on the height of the fog which they call Exponential Height Fog. This is an effect that looks really good, especially at larger scales like looking at mountain ranges and valleys.

I think CesiumJS could benefit a lot from this in terms of visual quality but I don't know how hard it is to implement.