Use per-broker secrets with StrimziPodSets

[scholzj]

In the past with StatefulSets, it was not easy to use different Secret or ConfigMap with each pod as they were all mounted through .spec.template in the StatefulSet resource. But now with StrimziPodSets, it is possible to do this. We already redesigned the configuration ConfigMaps for this. We should do the same also for the Secrets (e.g. the broker certificates secret) to better separate the different brokers.

This might not make sense for ZooKeeper since it will be gone at some point anyway. But it makes sense for Kafka which is to stay with us for the years to come ;-).

[scholzj]

Triaged on 12.1.2023: Should be looked into.