Support alpha in Standard Surface to glTF PBR translation

From issue #2578:

### Alpha
**glTF docs:** [Metallic-Roughness Material](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#metallic-roughness-material) (in the docs alpha is the forth component of base color)
**Standard surface doc:** [3.3 Transparency](https://autodesk.github.io/standard-surface/#closures/transparency)
**gltf_pbr inputs:** `alpha` 
**standard surface inputs:** `opacity`