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.

Comienza revisando las pruebas de redes compartidas mencionadas en #10612 y el comportamiento actual de Source NAT. Define la funcionalidad compatible y el tratamiento correspondiente en la API y la UI; se considerará terminado cuando el comportamiento y el alcance posterior de diseño o implementación no dejen lugar a ambigüedades.

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