Added Support for Scalar Subqueries with Aggregate Function State Result

Why it matters

This feature enables users to use aggregate function states within scalar subqueries, allowing more advanced aggregation computations and more efficient query patterns by reusing intermediate aggregate states.

How to use it

Users can write scalar subqueries that return aggregate function state values and utilize these states in outer queries. For example, you can use aggregate function states like sumState in a scalar subquery and then apply sumMerge on the result.