Support context completion for kubectl config use-context

With this commit, kubectl is supported context completion for config
use-context.

```
$ ./kubectl config use-context
cluster01  minikube
```

/refs #42103, #29386

pkg/kubectl/cmd/cmd.go

@@ -170,6 +170,10 @@ __custom_func() {
             __kubectl_get_resource_node
             return
             ;;
+        kubectl_config_use-context)
+            __kubectl_get_contexts
+            return
+            ;;
         *)
             ;;
     esac