cmd/{k8s-proxy,containerboot,k8s-operator},kube: add health check and metrics endpoints for k8s-proxy (#16540)

* Modifies the k8s-proxy to expose health check and metrics
endpoints on the Pod's IP.

* Moves cmd/containerboot/healthz.go and cmd/containerboot/metrics.go to
  /kube to be shared with /k8s-proxy.

Updates #13358

Signed-off-by: David Bond <davidsbond93@gmail.com>

cmd/k8s-operator/proxygroup_test.go

@@ -1379,6 +1379,8 @@ func TestKubeAPIServerType_DoesNotOverwriteServicesConfig(t *testing.T) {
 				Mode:       ptr.To(kubetypes.APIServerProxyModeNoAuth),
 				IssueCerts: opt.NewBool(true),
 			},
+			LocalPort:          ptr.To(uint16(9002)),
+			HealthCheckEnabled: opt.NewBool(true),
 		},
 	}
 	cfgB, err := json.Marshal(cfg)