sq key metadata

We should have a way to add and remove meta-data from a binding signature. sq key metadata list could list meta-data on a component's active binding signature, sq key metadata modify --add-notation A=B --remove-notation C --set-features ... could be used to modify them. (We want to batch the operations to avoid unnecessarily creating binding signatures.) I think it would be nice if sq key metadata modify also supported a --dry-run option to show the changes.