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.

#10612で参照されている共有ネットワークのテストと、現在のSource NATの動作を確認することから始めます。サポートする機能と、それに対応するAPIおよびUIでの扱いを定義します。動作と、その後の設計または実装の範囲が明確になれば完了です。

索引モデルが issue の本文から書いたものです。