invent a way to refer to additional manifest annotations via a URI

[tych0]

Right now, we are using tools like containers/image to transport images and umoci copy viz. #359 will be awesome to use to replace this.

One problem we have with containers/image is that it can't refer to any annotations besides org.opencontainers.image.ref.name, so we end up adding lots of metadata there and parsing it with ugly hacks insteading using something more structured like e.g.additional annotations on the manifest tha
t org.opencontainers.image.ref.name points to.

What would be awesome is if we could select images based on some URI; for
example:

umoci copy "oci:myimage?com.cisco.product.version=x.y.z&com.example.other .annotation=foo" $dest, which would select all images that have both name == myimage and match the other annotations too.

We could add some flags like umoci copy oci:myimage --addl-annotation com.example.other.annotation=foo $dest, but it does seem useful to be able t
o refert to an image with a single URI.

Thoughts?