[REFACTOR] Move `secret_metadata` function to WrappedSealedBox
[REFACTOR] Move `secret_metadata` function to WrappedSealedBox

This is better understood as a method attached to `WrappedSealedBox`.

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat