Add method(s) for unversioned labels

[adwk67]

For listener-class integration, labels are owned by the listener-operator (if the operator is responsible for creating them).
e.g. the Airflow operator has:

    // Used for PVC templates that cannot be modified once they are deployed
    let unversioned_recommended_labels = Labels::recommended(build_recommended_labels(
        airflow,
        AIRFLOW_CONTROLLER_NAME,
        // A version value is required, and we do want to use the "recommended" format for the other desired labels
        "none",
        &rolegroup_ref.role,
        "none",
    ))
    .context(LabelBuildSnafu)?;

This should be moved to operator-rs.