Add support for custom Service labels via spec.service.labels

[EladAviczer]

This closes #1821

Summary Of Changes

This PR introduces support for applying custom labels to the Service created by the RabbitmqCluster resource.
in the practicesuggested by the issue opener spec.service.labels

Additional Context

Local Testing

unit and integration tests succeeded.
Deployed to local cluster using the makefile.

Sample usage

apiVersion: rabbitmq.com/v1beta1
kind: RabbitmqCluster
metadata:
  name: example
spec:
  replicas: 1
  service:
    labels:
      foo: "bar"

[EladAviczer]

For the reviewer: please consider moving the if clause further down in the function for aesthetic reasons