Add list and inspect methods - cleanup hf_api
Add a bunch of methods to easily list and inspect the processing scripts up-loaded on S3:
```python
nlp.list_datasets()
nlp.list_metrics()
# Copy and prepare the scripts at `local_path` for easy inspection/modification.
nlp.inspect_dataset(path, local_path) 
# Copy and prepare the scripts at `local_path` for easy inspection/modification.
nlp.inspect_metric(path, local_path) 
```

Also clean up the `HfAPI` to use `dataclasses` for better user-experience