Improve tools/get_image_refs.py to identify which Sylva unit uses which OCI artifact.

With !5434 (merged), the script identifies OCI artifacts used in a cluster. We need to extend this to associate each OCI artifact with the Sylva unit(s) that use it.
The sylva-units.unit label present on Kustomizations and HelmReleases can be used for that.

This will be useful for air-gapped deployments and supply chain management

---

In the meantime, it might be a good idea to replace the OCI URL (which may include proxy information) with the actual upstream URL. We can retrieve this information from the values.

$ kubectl get secrets sylva-units-values -o go-template='{{.data.values|base64decode}}' | yq .sylva_base_oci_registry                                           
oci://172.20.136.39/proxy_cache_registry.gitlab.com/sylva-projects