Implement Exponential Height Fog


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](https://dev.epicgames.com/documentation/en-us/unreal-engine/exponential-height-fog-in-unreal-engine). 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.

No CesiumJS files, tests, or entry points are identified. Start by reviewing issue #12126 and the linked Unreal Engine Exponential Height Fog reference; the implementation scope and verification criteria still need to be established.

Written by the indexing model from the issue text.