Add support for a "postgres_query" function that allows you to run arbitrary queries against postgres

[Mytherin]

Implements #78

The postgres_query builds on the new attach functionality, and takes a reference to a currently attached Postgres database to run the query within. Signature:

postgres_query(attached_database::VARCHAR, query::VARCHAR)

Example:

D ATTACH 'dbname=postgresscanner' AS s (TYPE POSTGRES);
D SELECT * FROM postgres_query('s', 'SELECT * FROM cars LIMIT 3');
┌──────────────┬───────────┬─────────┐
│    brand     │   model   │  color  │
│   varchar    │  varchar  │ varchar │
├──────────────┼───────────┼─────────┤
│ ferari       │ testarosa │ red     │
│ aston martin │ db2       │ blue    │
│ bentley      │ mulsanne  │ gray    │
└──────────────┴───────────┴─────────┘