Allows management of referrer objects when performing
pull, archive export and archive import.
Referrer objects are linked to their subjects via GC
labels. The label is based on sha256 checksum of the
object instead of incremental numbers as referrers are
not immutable and don't have any strict order.
In OCI-layout referrers that are not already in the exported
tree are added to the main index.json with
io.containerd.manifest.subject annotation.
On import such descriptors with that annotation
do not create digest-based images in the image store.
Note that this does not mean all the referrer objects in
the registry are now pulled/exported/imported by default.
The caller of the client pkg functions can choose which
referrer objects should also be handled.
Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Tonis Tiigi