(feat) Implement Metallic/Roughness scale sliders

[rhammond17]

Implement the Metallic and Roughness scale sliders.

This is accomplished by adding Math-Multiply nodes between the metallic_sep and bsdf nodes.

Note: This change adds new nodes to the shader graph, it will not update already existing materials. To use this new feature, a new material will need to be created.

This is my first foray into the Blender API, please let me know if I could have done something a better way.

[tml1024]

As I try to explain in the repo's README.md, I don't really know anything about the code of Blender2MSFS2 myself, I just set up the repository. So I am not able to review this. Unless somebody else opposes, I will merge this PR after a few days.

Personally I hope that the "official" Blender add-on at https://github.com/AsoboStudio/glTF-Blender-IO-MSFS will be ready in some few months, and then this Blender2MSFS2 will not be needed any longer.

[btronquo]

Hello and thanks for your work on this,
However, IMO, at this moment, there is no goal to add features to this project since it will be deprecated in few weeks/months
Based on the progress of the official Asobo Blender Pluging, I think the first release of this plugin will be coming sooner that expected (I hope so !)

[rhammond17]

When I posted this PR, I also sent it to a friend. He then told me about the official glTF-Blender-IO-MSFS plugin which I hadn't known existed. I checked it out and they already have the sliders working. Ah well, still a good learning experience.

[rhammond17]

The new plugin is "mostly" working from my friend. There may be a few things missing and the GUID generation isn't implemented yet as far as I can tell, so this plugins isn't quite deprecated yet... However you might be able to copy the XML file with the GUID from this exporter to the directory with the new exporter files.