When not using an AD domain, create a private DNS zone for VM record registration

The private DNS zone name will be a new parameter and an example could be `research.aelterman.cloud`. Auto-registration will be enabled for virtual networks linked to this zone.

This needs to be created in the hub and linked to both the hub and the spoke networks.

Start by locating the repository's Bicep entry point for the hub-and-spoke network and identify how parameters and virtual-network links are defined. Add the private DNS zone name parameter, create the zone in the hub, enable auto-registration, and link it to both hub and spoke networks. Done means deployments without an AD domain register VM records through the new zone.

Written by the indexing model from the issue text.