Support creating tables from SELECT statements

[chaas]

Right now, we only support creating tables statically like:

CREATE TABLE example (a int, b text NOT NULL);

It would be great to support creating tables from SELECT statements, like:

CREATE TABLE example AS SELECT 1;

Postgres supports this:

postgres=# CREATE TABLE example AS SELECT 1;
SELECT 1
postgres=# SELECT * FROM example;
 ?column?
----------
        1
(1 row)

This would make our table functionality more flexible. As a bonus, it would allow us to easily create tables using dbt (dbt expects materializations to be created from SELECT statements).