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)

No se especifican archivos de implementación ni pruebas. Empieza reproduciendo el fallo mediante los pasos indicados de network-offering y isolated-network; después, lee la discusión relacionada #13052 y rastrea la validación de la obtención de la IP pública. Se considera completado cuando se puede añadir una IP pública a una red aislada avanzada con Static NAT y sin Source NAT, sin romper el comportamiento existente de Source NAT.

Escrito por el modelo de indexación a partir del texto del issue.