Add managed firewall support for VKE

[Byteflux]

Description

Adds support for the firewall_group_id field that is now returned as part of VKE cluster responses.

Additionally adds support for enable_firewall which can be provided when creating a new VKE cluster. When enabled, a Firewall Group will be deployed and managed by the cluster, ensuring worker nodes are automatically linked to the firewall.

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]

The API doesn't seem to return anything in firewall_group but govultr pulls it in and sends out enable_firewall so I think this is working.