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.

首先查看 #10612 中引用的共享网络测试以及当前的 Source NAT 行为。定义受支持的功能以及相应的 API 和 UI 处理方式；当行为和后续设计或实现范围都明确无歧义时，即视为完成。

由索引模型根据 Issue 内容生成。