Define the proper functionality for source nat in shared networks

### The required feature described as a wish

As an implementer I would like to be clear on what the functionality of shared networks is completely and without ambiguity.

as shown in testing for #10612, shared networks are now allowed to support the functionality “Source NAT” , but as the VMs in a shared network have their own public IP, this does not make much sense. It would make sense if some plugin would provide “Firewall”, or “Packet Shaper”/“QoS" for the network however.

Investigation is needed (probably followed by design/implementation) to see what should be possible and how to deal with it in API and UI.

Beginne mit der Prüfung des in #10612 referenzierten Tests für gemeinsam genutzte Netzwerke und des aktuellen Source-NAT-Verhaltens. Definiere die unterstützte Funktionalität sowie die entsprechende Behandlung in API und UI; abgeschlossen ist die Aufgabe, wenn das Verhalten und der anschließende Design- oder Implementierungsumfang eindeutig sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.