sql/opt: push Lock below ProjectSet

When using the new lock operator, we add locking at the top of the query tree. One case where we should push locking down is when a project-set is between the lock op and the rest of the plan. For example:

```sql
CREATE TABLE ab (
a INT PRIMARY KEY,
b INT
);

INSERT INTO ab VALUES (2, 2);

-- when using the old lock plan, locking is on the initial scan
EXPLAIN
SELECT generate_series(0, b)
FROM ab WHERE a = 2
FOR UPDATE OF ab;

SET optimizer_use_lock_op_for_serializable = on;

-- when using the new lock plan, locking happens after the project set
EXPLAIN
SELECT generate_series(0, b)
FROM ab WHERE a = 2
FOR UPDATE OF ab;

Jira issue: CRDB-57890