fix two problems:
1. push agg-fun in windowExpression down to AggregateNode
for example, sql:
select sum(sum(a)) over (order by b)
Plan:
windowExpression( sum(y) over (order by b))
+--- Agg(sum(a) as y, b)
2. push other expr to upper proj
for example, sql:
select sum(a+1) over ()
Plan:
windowExpression(sum(y) over ())
+--- Project(a + 1 as y,...)
+--- Agg(a,...)
