Represent retro-reflective materials

Retro-reflective surfaces are ubiquitous, especially in scenes representing urban and industrial environment, as those materials are commonly used for any equipment related to safety.

There has been extensive discussion on slack (see [slack thread](https://academysoftwarefdn.slack.com/archives/C0230LWBE2X/p1724694612039929) and [slack thread](https://academysoftwarefdn.slack.com/archives/C06365EAQMP/p1726694408972459)) on the topic.

We have so far identified two possible approaches for representing retro-reflective materials in OpenPBR :
- New BSDF : see proposal [Retro Reflective Node.pdf](https://github.com/user-attachments/files/17922517/Retro.Reflective.Node.pdf) [shadertoy implementation](https://www.shadertoy.com/view/MXscW2)
- Modified NDF of the coating / main dielectric layer based on [BRDF Measurements and Analysis of Retroreflective Materials](https://hal.science/hal-01083366/) [shadertoy implementation](https://www.shadertoy.com/view/XX2czz)

There are additional techniques commonly used to implement retro-reflective surfaces, based on shading normal modifications.

References :
- https://www.researchgate.net/publication/323012340_A_retroreflective_BRDF_model_based_on_prismatic_sheeting_and_microfacet_theory
- https://www.researchgate.net/publication/317575122_A_physically-based_BRDF_model_for_retroreflection
- https://www.researchgate.net/publication/316656998_Simulation_of_small-_and_wide-angle_scattering_properties_of_glass-bead_retroreflectors
- https://www.researchgate.net/publication/251610177_Photographic_assessment_of_retroreflective_film_properties
- https://www.artstation.com/blogs/d4ku/wZrD/a-retro-reflective-shader-for-unity
- BRDF Measurements and Analysis of Retroreflective Materials https://hal.science/hal-01083366/
- A brief history of retroreflective sign face sheet materials https://www.rema.org.uk/pub/pdf/history-retroreflective-materials.pdf

Start with the linked Retro Reflective Node.pdf and its Shadertoy implementation, then compare them with the modified-NDF Shadertoy implementation and the cited BRDF papers. Resolve which representation OpenPBR should adopt, including its scope and validation approach; the issue is done when the design is agreed and specified.

Written by the indexing model from the issue text.