cmd/kube-apiserver: add .import-restrictions

Allow only /pkg, /plugin, /third_party, /cmd/kube-apiserver, /test/utils.
This disallows imports of other cmd packages like cmd/kubeadm.

cmd/kube-apiserver/.import-restrictions

@@ -0,0 +1,8 @@
+rules:
+  - selectorRegexp: k8s[.]io/kubernetes
+    allowedPrefixes:
+      - k8s.io/kubernetes/cmd/kube-apiserver
+      - k8s.io/kubernetes/pkg
+      - k8s.io/kubernetes/plugin
+      - k8s.io/kubernetes/test/utils
+      - k8s.io/kubernetes/third_party