Allow dynamic first dimension for ArrayXD
Add support for dynamic first dimension for ArrayXD features. See issue [#887](https://github.com/huggingface/datasets/issues/887).
Following changes allow for `to_pylist` method of `ArrayExtensionArray` to return a list of numpy arrays where fist dimension can vary.

@lhoestq Could you suggest how you want to extend test suit. For now I added only very limited testing.