Add support for Managed Database public/private hostnames

[christhemorse]

Description

We recently rolled out support for distinguishing between public and private hostnames for Managed Databases, in which the regular host field points to the private address of a service attached to a VPC network and the new public_host field will then provide the public address. This PR matches that API functionality for the Govultr library.

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]

Looks good to me!