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.

先檢視 #10612 中提到的共用網路測試，以及目前的 Source NAT 行為。定義支援的功能，以及相應的 API 和 UI 處理方式；當行為和後續設計或實作範圍都明確無歧義時，即表示完成。

由索引模型根據 Issue 內容生成。