Make Image cast storage faster

[Modexus]

PR for issue #6782.
Makes cast_storage of the Image class faster by removing the slow call to .pylist.
Instead directly convert each ListArray item to either Array2DExtensionType or Array3DExtensionType.

This also preserves the dtype removing the warning if the array is already uint8.

[HuggingFaceDocBuilderDev]

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[lhoestq]

Hi ! Thanks for diving into this, this conversion to python lists is indeed quite slow.

Array2DExtensionType and Array3DExtensionType currently rely on pyarrow lists, but we will soon modify them to use FixedShapeTensorArray instead which is more efficient (e.g. doesn't need to store an offset for each value). So ideally it would be cool to speed this code up without using those extension types or it will be blocking to improve Array2DExtensionType and Array3DExtensionType.

If I understand correctly you just need the logic from ArrayExtensionArray.to_numpy ? If so feel free to make a separate function and ArrayExtensionArray.to_numpy can call it