Add support for `CREATE TABLE AS` on postgresql

[pietermartin]

CREATE TABLE AS can be used like a view.
Currently in Sqlg the pattern is to execute the query with limit 1, get the headers, drop/create the table, then execute insert into select from

This requires executing the query 2 times, first time with limit 1 which can still take long with complicated queries.

Look at supporting doing this with on query using CREATE TABLE AS...