Allow public IP acquisition without Source NAT

Allow public IP acquisition without Source NAT in Isolated Networks with Static NAT

ISSUE TYPE
- Improvement Request

COMPONENT NAME
```
```
CLOUDSTACK VERSION
```
4.20.3
```
SUMMARY

In an isolated network without Source NAT, it is not possible to add a public IP address when Static NAT is enabled.

Error:
```
Failed to acquire IP.
In zone of type Advanced ip address can be associated only to the network of guest type Isolated with the SourceNat enabled
```

It would be useful to have this capability, since Source NAT reserves a separate public IP address for itself. In scenarios where all VMs are expected to use Static NAT, the Source NAT IP remains idle and unused.

STEPS TO REPRODUCE
1) Create network offering for guest isolated network
2) Enable Static NAT in the offering
4) Create an isolated network using the created offering
5) Add a public IP address

Related discussion: [Error when adding a public IP in an isolated network without Source NAT (using Static NAT) #13052](https://github.com/apache/cloudstack/discussions/13052)

未指定實作檔案或測試。首先按照列出的 network-offering 和 isolated-network 步驟重現故障，接著閱讀相關討論 #13052，並追蹤公用 IP 取得驗證流程。完成的標準是：可以將公用 IP 新增至啟用 Static NAT 且不使用 Source NAT 的進階隔離網路中，同時不破壞現有的 Source NAT 行為。

由索引模型根據 Issue 內容生成。