sample-controller: add status subresource support

- Add an example to show how to use status subresources
with custom resources.
- Update the comment in the controller to mention that
`UpdateStatus` can now be used.
- Generate `UpdateStatus` for Foo.
- Update the README to remove feature gate information for
CRD validation since the current example requires a v1.9
cluster and it is enabled by default.
- Update the README to add feature gate information for
CustomResourceSubResources.

Kubernetes-commit: 7c06d6fb17ee46f587c1facff93b9bb185522855

artifacts/examples/crd-status-subresource.yaml

@@ -0,0 +1,13 @@
+apiVersion: apiextensions.k8s.io/v1beta1
+kind: CustomResourceDefinition
+metadata:
+  name: foos.samplecontroller.k8s.io
+spec:
+  group: samplecontroller.k8s.io
+  version: v1alpha1
+  names:
+    kind: Foo
+    plural: foos
+  scope: Namespaced
+  subresources:
+    status: {}