Add support for local OCI Images

oc-mirror currently only allows for mirroring of images from image registries. 

This issue adds support for mirroring images from a local OCI layout. 

1. In the imageset config, if an image reference omits the URI prefix, default to docker://
2. in the imageset config, add support for image reference by URI prefix oci://
3. in the imageset config, all image reference fields should support local layout; including operator catalogs and ocp releases. 

See: https://github.com/openshift/oc-mirror/issues/446