Add sky controller prefab for sun positioning

[argallegos]

This implements a sun, sky, and atmosphere solution for Cesium for Unity per #173.
Includes the following components:

• A sky controller script that exposes time of day and other settings to the user to control direction of the sun
• A prefab that users can add to their levels to quickly set the required parameters
• A skybox shader for URP that creates an atmosphere blending effect when the camera moves from earth to space.

It's currently a work in progress. Remaining items:

• Ensure HDRP compatibility - this is primarily on the shader side, but further work needs to be done to tie the prefab to the HDRP atmosphere options.
• Implement sun positioning math in cesium-native
• Sky shader
  • Improved atmospheric blending
  • A ground color that reduces the visual impact of missing tiles

[flankechen]

looking forward for the implementation. is this branch merge into main or any tags now?
I try 1.7.1 sample sky-controller branch, not working. any idea of which sample branch I should use if going to try this?

[kring]

It's not available in any pre-built binaries. If you want to use this, the only option is to build from this PR's branch yourself. It's based on a very old version of Cesium for Unity, though, so you'll probably want to merge in main. In short, it's likely to be a lot of work. Sorry we don't have a simple solution right now.

[flankechen]

It's not available in any pre-built binaries. If you want to use this, the only option is to build from this PR's branch yourself. It's based on a very old version of Cesium for Unity, though, so you'll probably want to merge in main. In short, it's likely to be a lot of work. Sorry we don't have a simple solution right now.

I did try to build cesium from this PR branch and try to run in some cesium example branches, sadly not working. yeap, this looks some old codes. I might not have time for a more deep dive into this. Anyway, looking forward for this to develped and merge into main branch and get release. UE sky looks cool, looking forward for Unity.