USD export: prototype invoking Python chasers

This commit allows invoking user-defined Python 'hook' functions to extend
the USD export functionality.

Added support for registering subclasses of a new bpy.types.USDHook
type which may implement the hooks as member functions.  Supported
hook functions are on_export() and on_material_export().  Also added
definitions and Python registration for USDSceneExportContext and
USDMaterialExportContext structs that encapsulate arguments
to these functions.

Pull Request: https://projects.blender.org/blender/blender/pulls/108823

scripts/modules/bpy_types.py

@@ -942,6 +942,10 @@ class KeyingSetInfo(StructRNA, metaclass=RNAMeta):
     __slots__ = ()
 
 
+class USDHook(StructRNA, metaclass=RNAMeta):
+    __slots__ = ()
+
+
 class AddonPreferences(StructRNA, metaclass=RNAMeta):
     __slots__ = ()