Support KHR_materials_pbrSpecularGlossiness

Unity honours `KHR_materials_pbrSpecularGlossiness` on scene and wearable glTFs, taking precedence over metallic-roughness (`DecentralandMaterialGenerator.cs:41-52`). bevy_gltf doesn't include it in its extension set: content listing it in `extensionsRequired` is rejected outright, and otherwise it's silently ignored with the material falling back to metallic-roughness defaults (usually visibly wrong). Options: convert to metallic-roughness at load (the extension is archived upstream and has a documented conversion), or extend the gltf loader.