Allow `list_datasets` to include private datasets
Thanks for opening this issue :)

If it can help, I think you can already use `huggingface_hub` to achieve this:
```python
>>> from huggingface_hub import HfApi
>>> [ds_info.id for ds_info in HfApi().list_datasets(use_auth_token=token) if ds_info.private]
['bigscience/xxxx', 'bigscience-catalogue-data/xxxxxxx', ... ]
```

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Though the latest versions of `huggingface_hub` that contain this feature are not available on python 3.6, so maybe we should first drop support for python 3.6 (see #4460) to update `list_datasets` in `datasets` as well (or we would have to copy/paste some `huggingface_hub` code) Great, thanks @lhoestq the workaround works! I think it would be intuitive to have the support directly in `datasets` but it makes sense to wait given that the workaround exists :) i also think that going forward we should replace more and more implementations inside datasets with the corresponding ones from `huggingface_hub` (same as we're doing in `transformers`)
I am working with a large collection of private datasets, it would be convenient for me to be able to list them.

I would envision extending the convention of using `use_auth_token` keyword argument to `list_datasets` function, then calling:

```
list_datasets(use_auth_token="my_token")
```

would return the list of all datasets I have permissions to view, including private ones. The only current alternative I see is to use the hub website to manually obtain the list of dataset names - this is in the context of BigScience where respective private spaces contain hundreds of datasets, so not very convenient to list manually.