Add support for multiple expressions in order by clause for aggregate functions

[mustafasrepo]

sqlparser can parse query below successfully,

SELECT ARRAY_AGG(x ORDER BY x) AS a FROM T

However, it cannot parse query below

SELECT ARRAY_AGG(x ORDER BY x, y) AS a FROM T

It assumes that order by will contain single entry. However, second form is valid also. Postgres can run queries in the second form.
Add support for multiple expression in the order by clause for aggregate functions.