[Feature] Add ColorRamp

[denghilbert]

Description

This PR adds ColorRamp, similar in Blender to Mitsuba. The current implementation only supports RGB or Monochrome color representation, as Blender did. All the interpolation methods used in Blender are implemented and tested.

Testing

Many local tests use Blender as GT for comparison (since there is no Blender on the server).

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I cleaned the commit history and removed any "Merge" commits
• I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license