Support for KHR_materials_specular

**Is your feature request related to a problem? Please describe.**
Khronos has ratified the specular extension which supersedes the KHR_materials_pbrSpecularGlossiness extension. It would be good to have support for this.

**Describe the solution you'd like**
Filament(gltfio) should be able to load gltf models with KHR_materials_specular extension

**Describe alternatives you've considered**
A hacky translation to the supported KHR_materials_pbrSpecularGlossiness, but this will result only in partial support for the specular extension and not its full capability

**OS and backend**
N/A