Operator to prevent more than one proxy for the same workload

By mistake, I created two AuthProxyWorkload resources pointing to the same Deployment, and it basically blocked my ReplicaSet from working, because the environment variables set for the `host` and `port` were already set for that workload.
It took me a while to find out about the second AuthProxyWorkload, although the operator was smart enough to block having the env vars twice, the error was not clear enough.
I'd like to propose that the operator reject the AuthProxyWorkload when it detects that a resource already points to the same workload.