feat(istioctl): add plugin for istioctl (#10410)

* Add plugin for istioctl completion. close #10405

* fix typo

* Update README.md

plugins/istioctl/README.md

+# Istioctl - Istio Command-line Tool
+
+The [istioctl](https://istio.io/latest/docs/ops/diagnostic-tools/istioctl/) tool is a configuration command line utility that allows service operators to debug and diagnose their Istio service mesh deployments.
+
+To use it, add `istioctl` to the plugins array of your zshrc file:
+
+```sh
+plugins=(... istioctl)
+```

plugins/istioctl/istioctl.plugin.zsh

+if [ $commands[istioctl] ]; then
+  source <(istioctl completion zsh)
+  compdef _istioctl istioctl
+fi