extend cluster-garbage-collector to cleanup Metal3DataTemplates

The following discussion from sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster!459 (merged) should be addressed as a follow-up to sylva-projects/sylva-elements/helm-charts/sylva-capi-cluster!459 (merged) :

• @tmmorin started a discussion:

  This MR will require follow-up work in the cluster-garbage-collector unit to ensure that Metal3DataTemplate are cleaned up when not referred to anymore by any resource.