add support for subqueries in select expression

[sasa-b]

This is a common use case when we want to have a column derived from a subquery, most often it is a correlated subquery https://dev.mysql.com/doc/refman/8.4/en/correlated-subqueries.html.

SELECT 
departments.id,
depratments.name,
(SELECT COUNT(*) FROM employees WHERE employees.department_id = departments.id) AS total_employess 
FROM departments;

P.S. If there is interest, I can raise a follow-up PR to add support for subqueries in where statements.

This is kinda already supported with the InListExpression and ExistsExpression, but that only covers two use cases, we could add support for all the other operators like >, <, =, >=, <= etc. It's fairly common to have aggregate subqueries in where conditions.