cmd/oci-image-tool: support multiple --ref

[runcom]

Adds the possibility to validate multiple reference at once:

$ oci-image-tool validate --ref tag --ref 3.1 --ref 12.04 --ref latest --ref notlatest /home/amurdaca/src/github.com/docker/docker/test
reference "tag": OK
reference "3.1": OK
reference "12.04": OK
reference "latest": OK
reference "notlatest": OK
/home/amurdaca/src/github.com/docker/docker/test: OK

$ oci-image-tool validate --ref tag --ref 3.1 --ref 12.04 --ref notexists --ref latest --ref notlatest /home/amurdaca/src/github.com/docker/docker/test 
reference "tag": OK
reference "3.1": OK
reference "12.04": OK
/home/amurdaca/src/github.com/docker/docker/test: validation failed: refs/notexists: descriptor not found

/cc @RobDolinMS @s-urbaniak @philips @vbatts

Signed-off-by: Antonio Murdaca runcom@redhat.com

[philips]

LGTM

[vbatts]

LGTM

[s-urbaniak]

fwiw, lgtm too ;-)