Support defining windowing behavior in the WINDOW clause #760

PostgreSQL and SparkSQL support defining the windowing behavior in the WINDOW clause and then referenced in OVER.
This is very useful if the same windowing behavior is wanted for several functions.

For example:

SELECT sum(salary) OVER w, avg(salary) OVER w
  FROM empsalary
  WINDOW w AS (PARTITION BY depname ORDER BY salary DESC);

https://www.postgresql.org/docs/15/tutorial-window.html

https://www.postgresql.org/docs/15/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS