Allow Setting Custom label on Service

[jseiser]

Is your feature request related to a problem? Please describe.
We have multiple external-dns configurations, so we use labels to inform which external-dns should handle creating the DNS for an Ingress/Service.

Describe the solution you'd like

apiVersion: vpn.example.com/v1alpha1
kind: Wireguard
metadata:
  name: "trex-ng-wg"
spec:
  serviceAnnotations:
    custom: label