feat: scalar subquery and correlated subquery

ref https://github.com/google/zetasql/blob/master/docs/subqueries.md

Currently only table subquery is supported, while in many cases, subquery inside expression is necessary.
E.g.
```sql
SELECT mascot
FROM Mascots
WHERE
NOT EXISTS(SELECT username FROM Players WHERE Mascots.team = Players.team);
```

or

```sql
SELECT id, agg_func(col1) over w from t1
window w as (
partition by col2 order by ts
rows between
(select max(col2) over w from t1 window w as(...) ) preceding
and current row
)
```