descriptor: add urls property
descriptor: add urls property

Signed-off-by: Antonio Murdaca <f9809039ea4850e75dcb8d80b778844aa463358f@redhat.com>

vbatts/oci-image-spec,opencontainers/image-spec,vbatts/oci-image-spec,opencontainers/image-tools,xiekeyang/image-tools,jonboulle/image-spec,mattspencer-arm/image-spec,opencontainers/image-tools,mattspencer-arm/image-spec,jonboulle/image-spec,opencontainers/image-spec,xiekeyang/image-tools