Before this patch, calling `image.Children` on an image built with
BuildKit would produce unnecessary `encountered unknown type
application/vnd.in-toto+json; children may not be fetched` debug logs,
because the media type is neither a known layer or config type.
Make the `image.Children` aware of the attestation layers and don't
attempt to traverse them.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
