Add long answer candidates to natural questions dataset
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4368). All of your documentation changes will be reflected on that endpoint.
This is a modification of the Natural Questions dataset to include missing information specifically related to long answer candidates. (See here: https://github.com/google-research-datasets/natural-questions#long-answer-candidates). This information is important to ensure consistent comparison with prior work. It does not disturb the rest of the format . @lhoestq @albertvillanova 