[Pg] Added support for postgres CTE Materialization options

[gp27]

This PR adds support for forcefully enabling or disabling materialized CTEs (postgres docs).
Materialized CTEs can impact query performance, and having explicit control allows for better optimization.

This feature was requested in issue #2318

Usage

I added an optional second parameter { materialized?: boolean } to $with, allowing queries to specify materialization behavior like this:

/* If the CTE is referenced more than once in the main query,
this will make sure that is executed only one time */
const sq = db.$with('sq', { materialized: true }).as(/*sub query*/);

/* When only a small part of the CTE output is needed and computing
the full subquery is expensive, it might be cheaper to force postgres
to duplicate the computation by inlining the CTE */
const sq = db.$with('sq', { materialized: false }).as(/*sub query*/);

Notes

If the PR is approved, this option will need to be documented. I’m unsure of the specific guidelines for documentation, so any pointers would be appreciated.