Proposal: dual specular roughness sliders with mix

[sharktacos]

Throwing out for discussion the idea of having two specular roughness sliders mixed by a lerp (typically using a mask texture for the mix), similar to Megascans materials in Unreal.

This can of course be achieved for instance with a ND_mix_float in materialX, but it could be nice to have it built into the material since it is so commonly used by lookdev artists. Thought I'd bring it up for possible consideration.