Supporting user-defined aggregate functions

### Summary

Supporting user-defined aggregate functions

### Additional context

I have a custom type and a user-defined aggregate function that works on that type.

```
CREATE TYPE public.amount AS (
number numeric,
currency text
);

CREATE FUNCTION public.sum(state public.amount[], current public.amount) RETURNS public.amount[]
LANGUAGE plpgsql
AS $$
DECLARE
found boolean = false;
i int = 0;
BEGIN
IF array_length(state, 1) IS NULL THEN
RETURN array[current];
END IF;

FOR i IN 1..array_length(state, 1)
LOOP
IF state[i].currency = current.currency THEN
state[i].number := state[i].number + current.number;
found := true;
EXIT;
END IF;
END LOOP;

IF NOT found THEN
state := array_append(state, current);
END IF;

RETURN state;
END;
$$;

CREATE AGGREGATE public.sum(public.amount) (
SFUNC = public.sum,
STYPE = public.amount[],
INITCOND = '{}'
);
```

Unsurprisingly, columns using the custom `amount` type are not recognized as aggregatable by the plugin. I couldn't find any mention of user-defined aggregate functions in the documentation so I assume they are not supported. Would it be easy to add support? Any simple workarounds here to add support?