oci-image-tool: check media types under refs/

[runcom]

Came across this and oci-image-tool wasn't warning me when a reference descriptor under refs/ wasn't containing a valid media type (especially because it was the empty string with my testing).

Should oci-image-tool take care of this as well?

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

[runcom]

(likewise I think a check of oci-layout to be version == the spec version tested against is required to be added to the tool)

[coolljt0725]

I +1 on this, I came across this when I oci-image-tool validate an old image layout which layer media type is still application/vnd.oci.image.serialization.rootfs.tar.gzip, I think it should throw out a warning or error

[philips]

Yes, oci-image-tool should error out on this.

[runcom]

took care of this at #262