Add long answer candidates to natural questions dataset
_The documentation is not available anymore as the PR was closed or merged._ Once we have added `long_answer_candidates` maybe it would be worth to also add the missing `candidate_index` (inside `long_answer`). What do you think, @seirasto ? Also note the "Data Fields" section in the README is missing the `long_answer` field.

Moreover, there is no instance example in "Data Instances" section. We could either make these fixes in this PR or in a subsequent PR. @albertvillanova I've added the missing fields and updated the README to include a data instance and some other things.  Great! I've made the updates to align the README. Please let me know if I missed anything. As there were many minor little fixes, I thought it would be easier to fix them directly. I think the loading script is OK now. If it is also validated by another datasets maintainer, I could run the generation of the pre-processed data and then merge this PR into master (once all the tests are green).

CC: @lhoestq  It looks good to me, thanks @seirasto ! I have merged the master branch, so that we include all the fixes on Apache Beam + Google Dataflow. Pre-processing is running!

Already finished for "dev" config:
```python
In [2]: ds = load_dataset("datasets/natural_questions", "dev")

In [3]: ds
Out[3]: 
DatasetDict({
    validation: Dataset({
        features: ['id', 'document', 'question', 'long_answer_candidates', 'annotations'],
        num_rows: 7830
    })
})
``` There is an issue while running the preprocessing for the "default" (train+dev) config. Train data files are larger than than dev ones and workers run out of memory.

I'm opening a separate issue to handle this problem: #4525 @seirasto is proposing uploading their preprocessed data files to our Datasets bucket.

I think @lhoestq can give a more informed answer about authentication requirements. Now that the data fiels are uploaded, can you merge the `main` branch into yours to re-trigger the CI @seirasto please ? :) Then I think we can merge if it's good for you @albertvillanova  Merge is done! I think someone needs to approve the CI to run :)  Can you run `make style` to fix the code formatting required by the CI please ? Thanks @albertvillanova! I've committed all your suggestions. The CI is green. I'm merging this PR.
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 