Add hair_chiang_bsdf closure definition from MaterialX

[fpsunflower]

Description

This just adds a closure definition so the MaterialX project can work on their OSL backend bindings for this new BSDF.

Tests

We don't have an implementation of this closure (nor would be it be easy to provide since testrender doesn't support hair fibers and this model won't work well on triangulated hair. We'll have to rely on MaterialX and renderers implementing this closure for testing.

Checklist:

• [x] I have read the contribution guidelines.
• [ ] I have updated the documentation, if applicable.
• [ ] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [x] My code follows the prevailing code style of this project. If I haven't already run clang-format v17 before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.

[fpsunflower]

@jstone-lucasfilm for vis.

[fpsunflower]

Related MaterialX issue for posterity: https://github.com/AcademySoftwareFoundation/MaterialX/pull/1968