Allow removal of IPs from ExternalPPool

[ghormoon]

Describe the problem/challenge you have
I'm using an ExternalIPPool like this, just with bunch of random public IP addresses. Sometimes i find out i need to migrate an IP address to another cluster/non-kube host.

apiVersion: crd.antrea.io/v1beta1
kind: ExternalIPPool
metadata:
  name: antrea-test
  namespace: antrea
spec:
  ipRanges:
  - start: 10.10.0.1
    end: 10.10.0.1
  - start: 10.10.0.2
    end: 10.10.0.2
  nodeSelector: {}

When trying # kubectl edit -n antrea ExternalIPPool/antrea-test and removal of one of the ip ranges, I get an error

error: externalippools.crd.antrea.io "antrea-test" could not be patched: admission webhook "externalippoolvalidator.antrea.io" denied the request: existing IPRanges [10.10.0.2-10.10.0.2] cannot be deleted

Simmilar usage works with metallb, so it would be nice if it worked on antrea too.

Describe the solution you'd like
Allow removal of ipRange from a list in the ExternalIPPool object.
Optinally, allow also modification of a range (to make it smaller if you use it with whole subnets and not specific IPs), but as discussed on Slack, that would be a bit more complicated.

The webhook should not allow removal of an IP that is currently assigned (optionally can try to assign another one, if the conditions allow it)

Anything else you would like to add?
Was discussed here https://kubernetes.slack.com/archives/CR2J23M0X/p1726837982588189

As a manual hack, temporarily removing "UPDATE" keyword in the webhook works, but of course is not a clean solution, because it disables it completely.