Feature: Inherit annotation from label (or the other way around)

Please, answer some short questions which should help us to understand your problem / question better?

• Which image of the operator are you using? no idea
• Where do you run it - cloud or metal? Kubernetes or OpenShift? AWS K8s
• Are you running Postgres Operator in production? yes
• Type of issue? feature request

Status quo

The Postgres Operator configuration already has options inherited_annotations and inherited_labels, which both work in a similar way: An annotation or label on the Postgresql resource gets copied to a same-named annotation/label on the child resources which it creates (e.g. statefulsets and pods).

At Zalando, we are using inherited labels component,environment,team-id, and inherited annotation zalando.org/owning-application (instead of inheriting the application label, that instead is set to a fixed value).
So users are currently asked to set both the application label and the zalando.org/owning-application annotation on their Postgresql resource to the same value, where one of them should be enough.

Feature request

There should be a way of defining an inheritance of an annotation from a label (and the other way around, for completeness).

Potential configuration syntax:

   kubernetes:
      inherited_annotations:
        - "zalando.org/owning-application"
      inherited_labels:
        - component
        - environment
      inherited_annotations_from_labels:
        - annotation: "zalando.org/owning-application"
          from_label: "application"
          conflict_priority: -1
      inherited_labels_from_annotations:
        - label: "example"
          from_annotation: "example.org/example-label"
          conflict_priority: 3

The conflict priority would be used in case there are multiple sources for one label/annotation (with the ones coming from inherited_annotations/labels having priority 0).
Of course we could also just say that it's an error if there are conflicting sources for the same annotation/label, might be simpler.

Start by locating the configuration and inheritance handling for inherited_annotations and inherited_labels. Define how annotation-to-label and label-to-annotation mappings resolve conflicting sources, then add coverage for the proposed configuration and conflict behavior; done means mapped metadata is copied to child resources as specified.