Support importing and pulling manifest lists

As an OpenShift user
I want to import manifest lists
so that I can deploy images from docker.io/library (and other repositories that have manifest lists) on heterogeneous clusters via image streams triggers.

Acceptance criteria

• Image streams spec has a new flag that enables new behavior.
• docker.io/library/nginx (or any other image that uses manifest lists) can be imported using `oc tag` or `oc import-image`.
• Once a new image is imported, image stream triggers should successfully deploy it using the manifest list SHA.
• For every child image an image.openshift.io object should be created.
• The imported manifest list can be deployed using image reference like `image-registry.openshift-image-registry.svc:5000/namespace/imagestream:tag`.
• The imported digest should be the digest of the manifest list.

Affected repositories

• openshift/api
• openshift/openshift-apiserver
• openshift/image-registry