Support features in metadata configs
Support features in metadata configs, like:
```
configs:
  - config_name: default
    features:
      - name: id
        dtype: int64
      - name:  name
        dtype: string
      - name: score
        dtype: float64
```

This will allow to avoid inference of data types.

Currently, we allow passing this information in the `dataset_info` (instead of `configs`) field, but this is not intuitive and it is not properly documented.

TODO:
- [ ] Document usage