Support abbe number in dielectric_bsdf

[AdrienHerubel]

OpenPBR and Standard Surface define a dispersion parametrization based on the abbe number. The parametrization exists on the reference MaterialX graphs (https://github.com/AcademySoftwareFoundation/OpenPBR/blob/b212785f1e35462853c7d455ae5a41f8ce71bf98/reference/open_pbr_surface.mtlx#L38), but is not used or connected to any node making it difficult for implementers to use.

The abbe number should be exposed on the dielectric BSDF and implemented in each target.