Add PostgreSQL support

[maxtruxa]

This PR adds support for PostgreSQL using the postgresql driver name.

Any help adding automated tests through .github/workflows/main.yml would be appreciated.

postgresql/postgresql.go is mostly a copy of mysql/mysql.go with relevant changes applied. Going forward there are a few possibilities:

• Refactor the MySQL and PostgreSQL drivers use a common base. Less code.
• Refactor the PostgreSQL driver to use pgx directly. Better performance.
• Leave everything as it is now.

[dopey]

Hey @maxtruxa, thanks a ton for the PR! We're definitely going to get this reviewed and merged. Unfortunately we're completely swamped before the holidays. I'm probably the one to review this (and write the github actions integration for a psql test db). I'll pick this up as soon as I'm done with current work.

In the mean time, we'd love to send you some swag as a "thank you" for taking the time to contribute. If you're interested, shoot me an email with your name/address (I'm at max@smallstep.com). Cheers 🍻