[Feature request] Add a feature to dataset
Maybe you can use `with_indices`?

```python
new_info = list of length dataset['train']

def some_function(indice, x):
    # adds new_info[index of x] as a field to x
    x['new_info'] = new_info_x[indice]
    return x

dataset['train'] = dataset['train'].map(some_function, with_indices=True)
```
Is there a straightforward way to add a field to the arrow_dataset, prior to performing map?
[Feature request] Add a feature to dataset 
 Is there a straightforward way to add a field to the arrow_dataset, prior to performing map? 
 Maybe you can use `with_indices`?

```python
new_info = list of length dataset['train']

def some_function(indice, x):
    # adds new_info[index of x] as a field to x
    x['new_info'] = new_info_x[indice]
    return x

dataset['train'] = dataset['train'].map(some_function, with_indices=True)
```