[Multi-SLB] support ip sharing across multiple services

[nilo19]

What happened:

Service A has been created to use IP 1. Now we want to create a service B point to the same IP. During the lb selection phase of service B, it will select another LB with less existing services than service A's LB. Since the LB selected is a different one, LB migration logic will be triggered, and the service A's frontend IP config will be removed by accident.
https://github.com/kubernetes-sigs/cloud-provider-azure/blob/master/pkg/provider/azure_loadbalancer.go#L846

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• Kubernetes version (use kubectl version):
• Cloud provider or hardware configuration:
• OS (e.g: cat /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Network plugin and version (if this is a network-related bug):
• Others:

[nilo19]

Fix A:

1. Only do anything special when the Service pins a frontend.
2. Add a tiny “frontend → hosting LB” resolver, used only by getAzureLoadBalancerName.
3. Change only the multi-SLB branch in getAzureLoadBalancerName.
4. If currentLBName == "":
   • call the resolver:
     • if internal: use internal resolver
     • else: use external resolver (may need a pip LIST, but should be cached?)
   • if it returns hostingLBName != "" and it matches one of eligibleLBs (case-insensitive):
     • set currentLBName to the exact eligible entry (so existing StringInSlice checks work).
5. Call getMostEligibleLBForService(currentLBName, ...) as today.

[nilo19]

Fix B:

1. Add isPrimaryService to return values of getServiceLoadBalancerStatus.
2. Skip LB migration for secondary services.

[nilo19]

Report an error when the service LB selection does not match the one containing the IP it tries to use.

[Liunardy]

/assign