Allow additional allowed IPs for VMs in Security Group networks

### The required feature described as a wish

Hi,

I’m currently experimenting with Anycast for Kubernetes clusters.

For this setup, I have two BGP router VMs that talk to multiple tenant worker nodes. The worker nodes should be able to announce external IPs for Kubernetes LoadBalancer services.

The goal is to have multiple tenants on a shared L2 network and use Security Groups for isolation.

The issue is that CloudStack does not know about these external IPs in my scenario, so traffic gets blocked on the hypervisor iptables layer. As far as I understand it, only IPs that CloudStack knows about are allowed as destination IPs for a specific VM.

Even if I create a separate network containing those external IPs, I cannot assign the same IP to multiple VMs. Because of that, real Anycast is not possible.

What would be useful is a way to allow additional destination IPs for a VM or Security Group without requiring CloudStack to assign those IPs directly to a single VM. This would allow external systems, like Kubernetes with BGP-based LoadBalancer announcements, to handle the IP ownership dynamically while CloudStack still provides L2 isolation and anti-spoofing.

首先審查 issue 中描述的 Security Group 行為和 hypervisor iptables 過濾，重點關注目前如何為 VM 允許目的地 IP。定義如何在保留 L2 隔離和 anti-spoofing 的同時，為 VM 或 Security Group 允許其他目的地 IP，然後針對 Kubernetes BGP Anycast 情境驗證該設計。

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