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.

Start by reviewing the shared-network testing referenced in #10612 and the current Source NAT behavior. Define the supported functionality and the corresponding API and UI treatment; done means the behavior and follow-up design or implementation scope are unambiguous.

Written by the indexing model from the issue text.