Add uiname field to .mtlx nodedef

[portsmouth]

Since the Maya team noted that they need this field in order to derive the node name (with correct capitalization and word break) to show in the editor:

since the name of the node is open_pbr_surface, it will require custom code to turn it into "OpenPBR Surface".

uiname metadata is already in the MaterialX specifications in the "Custom Node Declaration NodeDef Elements" section for MaterialX 1.39 and I see this as well in the old 1.38 specification. Feel free to add it to OpenPBR surface and I will make sure LookdevX uses it.

[AdrienHerubel]

LGTM

[jstone-lucasfilm]

Thanks for this fix!