glTF importer: Implement a user extension system

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/imp/gltf2_blender_animation_node.py

io_scene_gltf2/blender/imp/gltf2_blender_animation_weight.py

io_scene_gltf2/blender/imp/gltf2_blender_camera.py

io_scene_gltf2/blender/imp/gltf2_blender_image.py

io_scene_gltf2/blender/imp/gltf2_blender_light.py

io_scene_gltf2/blender/imp/gltf2_blender_material.py

io_scene_gltf2/blender/imp/gltf2_blender_mesh.py

io_scene_gltf2/blender/imp/gltf2_blender_node.py

io_scene_gltf2/blender/imp/gltf2_blender_scene.py

io_scene_gltf2/blender/imp/gltf2_blender_texture.py

io_scene_gltf2/io/imp/gltf2_io_gltf.py

io_scene_gltf2/io/imp/gltf2_io_user_extensions.py