leverage managedFields

[itaysk]

interesting development in Kubernetes 1.18 - server side apply was rearchitected to track field ownership using a metadata field called managedFields. This field can be a great insight for kubectl-neat, especially when it comes to controllers, where it was hard to tell if the field was added by a controller (the should be removed), or by the user (and should be preserved) (For example: #12). we can now use the managedFields field get this information.
Another approach can be to just trim everything that's not managed by the user.