Loosen up the oci url to support custom built images

What does this MR do and why?

Adds a new variable sylva_images_oci_registries that will hold custom repositories hosting sylva-diskimage-builder oci artifacts which can be used by the generate-os-images helper to generate the url. The registry can then be specified for each sylva_diskimagebuilder_images.X.sylva_images_oci_registry.

This allows to use custom repos in the case someone wants to produce different images and needs to provide a different url to expose the artifacts.

Reduces:

os_images:
  my-custom-plain-rke2-1-27-6:
    uri: "oci://10.0.0.2/caas-images/my-custom-plain-rke2-1-27-6:0.1.1"
    filename: my-custom-plain-rke2-1.27.6.raw
    md5: 9c59d5cef96888cfe72bac8531e201e3
    sha256: 4b56645793121a63332b75db5228f75c6484daad8e24bb7f2566a49ad462de54
    image-format: raw
    flavor: plain
    k8s-flavor: rke2
    k8s-version: 1.27.6-rke2r1

to

sylva_diskimagebuilder_images:
  my-custom-plain-rke2-1-27-6:
    enabled: true
    sylva_images_oci_registry: custom
sylva_images_oci_registries:
  custom:
    url: oci://10.0.0.2/caas-images
    tag: 0.1.1