Support server-side dry run

Cherry-pick from HNC v0.5 - commit ce98366. Tested: many e2e tests are
currently broken on master (kubernetes-retired#1058) but the ones that are passing before
this change also pass after it.

Original commit message follows.

---

See issue kubernetes-retired#1027. To support server-side dry-run, all admission webhooks
must state that they do not have any side effects. Our version of
controller-gen doesn't include the sideEffects marker so I added them as
a kustomization patch.

Tested: server-side dry-run fails on a pod creation in Kind 1.18 without
this fix and passes with it. GKE doesn't have 1.18 yet so I couldn't
test it there, but I also tested it on GKE 1.15 (the oldest currently
supported version) to ensure that it had no ill effects, and all e2e
tests work fine on both Kind 1.18 and GKE 1.15.

incubator/hnc/config/webhook/webhook_patch.yaml

@@ -6,3 +6,12 @@ metadata:
 webhooks:
 - name: objects.hnc.x-k8s.io
   timeoutSeconds: 2
+  sideEffects: None
+- name: subnamespaceanchors.hnc.x-k8s.io
+  sideEffects: None
+- name: hierarchyconfigurations.hnc.x-k8s.io
+  sideEffects: None
+- name: hncconfigurations.hnc.x-k8s.io
+  sideEffects: None
+- name: namespaces.hnc.x-k8s.io
+  sideEffects: None