[Feature request] Add a feature to dataset
Given another source of data loaded in, I want to pre-add it to the dataset so that it aligns with the indices of the arrow dataset prior to performing map.

E.g. 
```
new_info = list of length dataset['train']

dataset['train'] = dataset['train'].map(lambda x: some_function(x, new_info[index of x]))

def some_function(x, new_info_x):
    # adds new_info[index of x] as a field to x
    x['new_info'] = new_info_x
    return x
```
I was thinking to instead create a new field in the arrow dataset so that instance x contains all the necessary information when map function is applied (since I don't have index information to pass to map function).
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? 
 Given another source of data loaded in, I want to pre-add it to the dataset so that it aligns with the indices of the arrow dataset prior to performing map.

E.g. 
```
new_info = list of length dataset['train']

dataset['train'] = dataset['train'].map(lambda x: some_function(x, new_info[index of x]))

def some_function(x, new_info_x):
    # adds new_info[index of x] as a field to x
    x['new_info'] = new_info_x
    return x
```
I was thinking to instead create a new field in the arrow dataset so that instance x contains all the necessary information when map function is applied (since I don't have index information to pass to map function).