Item's `ext` interface should include the scientific extension

It's [allowed](https://github.com/stac-extensions/scientific?tab=readme-ov-file#scientific-citation-extension-specification) but it's not [there](https://github.com/stac-utils/pystac/blob/14a36942d395de47ebdb7f22477ce00c5a87348f/pystac/extensions/ext.py#L126-L197).