Add UV selector texture

[dvicini]

Description

This adds a "UV selector" texture. This texture allows to use additional UV sets, e.g., for using light maps. While one could implement a generic solution in the Texture base class, I was thinking that a standalone plugin might be the better solution at the moment (given that most usecases use only the standard UV set). Supporting multiple UV sets is a meaningful step for Mitsuba to be able to ingest a broader set of existing assets. The change is contained to this one plugin and therefore very non-intrusive. The only change beyond the plugin is the addition of an eval_attribute_2 API to the shape class.

I am open also to naming suggestions for the plugin, uv_set_selector is a bit lengthy maybe.

Testing

Added basic unit tests

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