Add MaterialX exporter

[JenusL]

Arnold 5.3 includes a new MaterialX API that's implemented in the other DCC plugins. From 5.3.0.0 release notes:

• MaterialX API: A new function AiMaterialxWrite() bakes the shader and other look properties for one or more shapes to a .mtlx file along with the description of the shaders and shading graphs (#7883), and AiMaterialxGetLookNames() gets the list of looks contained in a .mtlx document. (#8087)

Should we implement this? Would anyone use it in SItoA?

[caron]

We could implement it for fun but it is certainly not high priority.

[JenusL]

I'm seeing this workflow used more and more in the MtoA mailing list so maybe it's worth looking in to.