Support `Array` logical operator to transform subquery results into an array

[maropu]

In PostgreSQL, ARRAY is not a function but a syntax defined in parser definition. So, Spark-2.2 can't handle some queries for metadata handling. See: https://github.com/maropu/spark-sql-server/blob/ed3e1ed39f51568e0d3526c273771b3ff7b8acff/sql/sql-server/src/test/scala/org/apache/spark/sql/server/PsqlCommandsSuite.scala#L260