Review `kubectl diff` logic and potentially support something similar

[ankon]

https://kubernetes.io/blog/2019/01/14/apiserver-dry-run-and-kubectl-diff/

Being able to see a diff of what we have in the templates and what exists on the server seems very useful. Depending on how kubectl does this, we might be able to do something similar (in the worst case: get the current resources from the server, and do a somewhat-smart structure diff).