Add the ListDigest to the container Spec struct and all its copies so we
can store list digests when they are available and pass them on to the
appropriate osbuild stages, sources, and inputs.
Copy the value whenever a spec is moved to a different representation.
The ellipsis operator was used as a hack to not need to pass any details
as an argument, but it makes what the end object will actually look like
less obvious. It also makes it impossible to pass an array to details
without getting a nested array.
Fixes#2874
