Major Description
For complex expressions in window spec definition, we extract it and replace it with an AttributeReference (with an internal column name, e.g. "_w0").
We can reuse the extracted expressions to avoid generator unnecessary Window node.
For example: the logical plan of query
SELECT max(value) over(partition by (key) order by cast(ts as timestamp)) as w1, min(value) over(partition by (key) order by cast(ts as timestamp)) as w2 FROM t1
contains two Window node.
