Kubernetes v1.5, `kubectl apply` has had an alpha-stage `--prune` flag to support deleting previously applied objects that have been removed from the input manifest. This feature has remained in alpha ever since due to performance and correctness issues inherent in its design. This PR exposes a second, independent pruning alpha powered by a new standard named `ApplySets`. An `ApplySet` is a server-side object (by default, a Secret; ConfigMaps are also allowed) that kubectl can use to accurately and efficiently...

* Mark image as deliberately optional in v1 Container struct.  Many objects in the Kubernetes API inherit the container struct and only Pods require the field to be set. ([#48406](https://github.com/kubernetes/kubernetes/pull/48406), [@gyliu513](https://github.com/gyliu513))