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)

未指定实现文件或测试。首先按照列出的 network-offering 和 isolated-network 步骤复现故障，然后阅读相关讨论 #13052，并跟踪公共 IP 获取验证流程。完成的标准是：可以将公共 IP 添加到启用 Static NAT 且不使用 Source NAT 的高级隔离网络中，同时不破坏现有的 Source NAT 行为。

由索引模型根据 Issue 内容生成。