[Python] Is there a way to call a custom compute function on a table.group_by aggregation?

### Describe the usage question you have. Please include as many useful details as possible.

Is there a way to call a custom compute function on a table.group_by aggregation? If so, what should the custom function return?

I do not see an example in the documentation.
Edit: using pyarrow.

### Component(s)

Python

Start with the PyArrow Python documentation for table.group_by and its aggregation examples, then check the related implementation or API reference to determine whether custom compute functions are supported. Document the supported usage and the required return value only if the behavior is confirmed, with an example that answers the question.

Written by the indexing model from the issue text.