Add `order by` support in aggregate functions argument scope.

[igalklebanov]

Hey 👋

Following #779, we might want to allow chaining .orderBy where .distinct is invocable in eb.fn.agg.

Example aggregate functions where order by can be used inside the argument scope:

• MySQL's group_concat.
• PostgreSQL's array_agg.
• SQLite's group_concat.
• Oracle's collect.

[trompx]

Hi @igalklebanov, Had just the need for that feature. Any idea if this will be merged soon?