chore(python): Rename pivot aggregate_fn to aggregate_function

[stinodego]

Follow-up of #7032

I am not 100% sure about this one.

aggregate_function feels too long for an argument that should usually be called as a keyword argument.

I think agg is nice - it's short and is the same as groupby.agg, which takes similar input.

But I am open to better ideas.

[ritchie46]

We go from intially more explicit to less now. I think the function has merit in explaining the keyword. So either aggrefate_fn is ok, or proposed aggregate_function or if deemed too long agg_fn.

[stinodego]

Right, let's just be consistent and make it aggregate_function.

I thought the implicit link with the groupby.agg method was sort-of nice, but I think this is better in line with our naming conventions.