container: add ListDigest to the spec

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.

internal/cloudapi/v2/server.go

@@ -434,6 +434,7 @@ func generateManifest(ctx context.Context, workers *worker.Server, depsolveJobID
 			containerSpecs[i].LocalName = s.Name
 			containerSpecs[i].TLSVerify = s.TLSVerify
 			containerSpecs[i].ImageID = s.ImageID
+			containerSpecs[i].ListDigest = s.ListDigest
 		}
 	}