Merge pull request #53920 from apelisse/diff

Automatic merge from submit-queue (batch tested with PRs 53051, 52489, 53920). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Implement `kubectl alpha diff` to diff resources

`kubectl alpha diff` lets you diff your resources against live
resources, or last applied, or even preview what changes are going to be
applied on the cluster.

This is still quite premature, and mostly untested.

**What this PR does / why we need it**: 

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:
Clearly not ready for Release note.
```release-note
NONE
```

kubernetes/community#287

docs/.generated_docs

@@ -18,6 +18,7 @@ docs/man/man1/kube-apiserver.1
 docs/man/man1/kube-controller-manager.1
 docs/man/man1/kube-proxy.1
 docs/man/man1/kube-scheduler.1
+docs/man/man1/kubectl-alpha-diff.1
 docs/man/man1/kubectl-alpha.1
 docs/man/man1/kubectl-annotate.1
 docs/man/man1/kubectl-api-versions.1
@@ -115,6 +116,8 @@ docs/man/man1/kubectl-version.1
 docs/man/man1/kubectl.1
 docs/man/man1/kubelet.1
 docs/user-guide/kubectl/kubectl.md
+docs/user-guide/kubectl/kubectl_alpha.md
+docs/user-guide/kubectl/kubectl_alpha_diff.md
 docs/user-guide/kubectl/kubectl_annotate.md
 docs/user-guide/kubectl/kubectl_api-versions.md
 docs/user-guide/kubectl/kubectl_apply.md

docs/man/man1/kubectl-alpha-diff.1

@@ -0,0 +1,3 @@
+This file is autogenerated, but we've stopped checking such files into the
+repository to reduce the need for rebases. Please run hack/generate-docs.sh to
+populate this file.

docs/user-guide/kubectl/kubectl_alpha.md

@@ -0,0 +1,3 @@
+This file is autogenerated, but we've stopped checking such files into the
+repository to reduce the need for rebases. Please run hack/generate-docs.sh to
+populate this file.

docs/user-guide/kubectl/kubectl_alpha_diff.md

@@ -0,0 +1,3 @@
+This file is autogenerated, but we've stopped checking such files into the
+repository to reduce the need for rebases. Please run hack/generate-docs.sh to
+populate this file.