Hydra: export Blender shader nodes as MaterialX node graph

This adds initial support for rendering Cycles and EEVEE shaders in Hydra
render engines that support MaterialX. Not all nodes are currently
supported, see the detailed compatibility list in #112864.

Co-authored-by: Georgiy Markelov <georgiy.m.markelov@gmail.com>
Co-authored-by: Vasyl Pidhirskyi <vpidhirskyi@gmail.com>

Pull Request: https://projects.blender.org/blender/blender/pulls/111765

doc/python_api/examples/bpy.types.HydraRenderEngine.py

@@ -16,6 +16,9 @@ class CustomHydraRenderEngine(bpy.types.HydraRenderEngine):
     # Name of the render plugin.
     bl_delegate_id = "HdCustomRendererPlugin"
 
+    # Use MaterialX instead of UsdPreviewSurface for materials.
+    bl_use_materialx = True
+
     # Register path to plugin.
     @classmethod
     def register(cls):