Allow removal of IPs from ExternalPPool

[rajnkamr]

Note -
Original PR - #7168
Due to authentication changed for the user, not able to push to the same PR, so rebased and created another PR

Allow removal of IPs from ExternalPPool:
Case:
When a user updates an ExternalIPPool, the webhook needs to:

Detect if any IP ranges are being updated from the pool.
If Updated ranges have allocated IPs, reject the removal update.
Otherwise, allow the update.
Example:

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
• cidr: 10.10.1.0/28
  nodeSelector: {}
  if range "10.10.0.1-10.10.0.1" allocated but "10.10.0.2-10.10.0.2" not allocated then allow the removal of range or ips on "10.10.0.2-10.10.0.2" range.

Removal of IPs from Range will not allow if range have any allocated IP.
"10.10.0.1-10.10.0.1"

fixes: #6874