Add texture load hook to gltf extension handler

[beicause]

Objective

Fixes #21185

Solution

Add on_texture_load hook to GltfExtensionHandler. After parsing gltf textures, we iterate gltf extention handlers to let them load the parsed textures to image handles. Gltf extention handlers can choose to bypass it or load it. Once a extention handler loads it we use its result and skip the remaining handlers.

I removed the loading textures with IoTaskPool because we can't - at least &mut LoadContext cannot be accessed from multiple threads. But this probably doesn't matter because load_context.load_builder().load() is already deferred load and multi-threaded. This might only be helpful for loading embedded images.

Also add HdrTextureLoader and ExrTextureLoader for loading external gltf textures by recognizing file extension. But this is not ideal since asset loader cannot recognize paths or attempt to load and recognize assets.

Testing

TODO