Add support to set brokerGroup specific labels to kafka pods

[amuraru]

Is your feature request related to a problem? Please describe.

Currently the operator support setting kafka pods labels via propagateLabels field in the KafkaCluster spec. This way not only the pods but all the created resources will get the labels.
In addition to that it would be useful to have the ability to set kafka pods labels specific for each brokerConfigurationGroup, i.e. kafka_broker_group: group_name
For our use-case this can be used in Prometheus monitoring to capture the group/az for each broker as a label.

Describe the solution you'd like to see

Add a new optional field to KafkaCluster spec: brokerConfigGroups.<group>.brokerLabels that's appended to the current list of pod labels when set.

Describe alternatives you've considered

Currently we are making use of brokerConfigGroups.<group>.brokerAnnotations but this needs additional manipulation in Prometheus ServiceMonitor definition to transform a pod annotation to a metric label.

[LukasSlouka]

Also useful with network policies

[panyuenlau]

Hey @amuraru @LukasSlouka, this has been added to the operator, see this newly added section about how to configure the kafkacluster to set a group-specific label for the brokers