Export Diffuse BSDF

[nonl4331]

Is your feature request related to a problem? Please describe.
The current behaviour which ignores the color and roughness fields of diffuse BSDF materials is rather confusing.

Q: Why not use Principled BSDF?
A: It is documented that diffuse BSDF matches a lambertian BSDF at roughness 0 and Oren-Nayer BSDF at non 0 roughnesses. If you want those BSDF models in particular (for instance to create reference render images with those BSDFs) then it is clear how to do it with diffuse BSDF but is not with a Principled BSDF. While people can switch to principled BSDF on export this is annoying as you will need to change it back afterwards.

Describe the solution you'd like
I'd like to implement and maintain diffuse BSDF support, this issue is just so I can get a go ahead. I understand that while others (see #1488) have also run into this issue this still in a niche issue so I will implement and maintain this myself to not burden others.

Describe alternatives you've considered
Not implementing and continuing status quo means the potential confusion remains.
Another alternative would be #2390 but I believe that it is a suboptimal fix in this case as it requires a lot of manual intervention and still doesn't aleviate possible confusion.

[nonl4331]

I have create a PR #2449 which I feel addresses the above. I have decided against including the roughness field of the diffuse BSDF as I feel it goes against the principle of least astonishment.