feat(operator): Add LabelExt trait

[Techassi]

This trait enables adding one or multiple labels to any Kubernetes resource (through kube's ResourceExt trait).

This feature came out of stackabletech/secret-operator#634 which needs adding of labels to the default custom resources.

let mut tls_secret_class: v1alpha2::SecretClass =
    serde_yaml::with::singleton_map_recursive::deserialize(deserializer)?;

tls_secret_class
    .add_label(Label::instance("secret-operator").unwrap())
    .add_label(Label::managed_by("secret-operator", "secretclass").unwrap())
    .add_label(Label::name("secret-operator").unwrap())
    .add_label(Label::version(built_info::PKG_VERSION).unwrap())
    .add_label(Label::stackable_vendor());

[NickLarsenNZ]

LGTM

Thanks for fixing the doc-comment titles.

[NickLarsenNZ]

LGTM