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.

Commencez par examiner les tests de réseau partagé référencés dans #10612 et le comportement actuel de Source NAT. Définissez les fonctionnalités prises en charge ainsi que le traitement correspondant dans l'API et l'UI ; le travail sera considéré comme terminé lorsque le comportement et la portée des travaux ultérieurs de conception ou d'implémentation seront sans ambiguïté.

Rédigé par le modèle d'indexation à partir du texte de l'issue.