executor: implement SortAgg

[skyzh]

SortAgg is a special aggregator that takes an input of sorted group keys. For example:

select sum(x) from table group by group_key;

| group_key | x |
| a         | 1 |
| a         | 2 |
| a         | 3 |
| b         | 1 |

As data are already sorted with group_key, SortAgg can only have state for one group at a time, which significantly reduces memory footprint.

[Kikkon]

Let me have a try! : )