flatpak_oci_registry_load_blob: Allow tags (and digests) arg
flatpak_oci_registry_load_blob: Allow tags (and digests) arg

We typically only load by digest id, resolving tags in other ways.
However, we need to load the _deltaindex tag by tagname, so support
this when possible.

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak