Allow `list_datasets` to include private datasets
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.