Add support for DBaaS FerretDB offering

[christhemorse]

Description

This PR updates Govultr to support the new API properties associated with the FerretDB offering for Vultr Managed Databases. The API docs already support "ferretpg" as a valid engine type upon launch, but the database results need to support to ferretdb_credentials object (and associated connection info) now returned from the API for those subscription types.

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[optik-aper]

Good to go