Add support for glTF KHR_materials_anisotropy extension

[jjhembd]

Feature

The KHR_materials_anisotropy extension extends the metallic-roughness PBR model to account for anisotropic reflectivity. A surface with anisotropic reflectivity (such as a brushed metal) will reflect differently at different orientation angles in the plane of the surface (even for the same incident and reflected angles with the normal).

CesiumJS currently does not recognize these parameters in a glTF. Here is how the anisotropy test model is currently rendered. Note the lack of variation within columns:

Compare to the result from the reference implementation with the extension enabled: