Is your feature request related to a problem? Please describe.
Google Cloud DNS API supports "Private" dns zones, and can have multiple dns zones with the same dns name in the same project, enabling split horizon dns (https://cloud.google.com/dns/docs/zones/zones-overview#split-horizon). #2482 adds support for private dns zones, however the dnscontrol GCLOUD provider still cannot support split horizon dns within the same GCP project.

Describe the solution you'd like
Support split horizon domains within the same GCP project

Describe alternatives you've considered
Private and Public zones can be defined in different GCP projects to enable split horizon dns.

Additional context
In order to fully support split horizon domains within the same GCP project, need to parse the zone Visibility field from *ManagedZone in *gcloudProvider.loadZoneInfo(), but currently gcloudProvider.zones is map[string]*gdns.ManagedZone where the map keys are the zone dns names. A given GCP project can have multiple zones of the same dns name.