feat(fluxcd): add completion for flux (#11350)

plugins/fluxcd/README.md

+# FluxCD plugin
+
+This plugin adds completion for [FluxCD](https://fluxcd.io), an open and extensible continuous delivery solution for Kubernetes. Powered by GitOps Toolkit.
+
+To use it, add `fluxcd` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... fluxcd)
+```

plugins/fluxcd/fluxcd.plugin.zsh

+# Autocompletion for the FluxCD CLI (flux).
+if (( ! $+commands[flux] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `flux`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_flux" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _flux
+  _comps[flux]=_flux
+fi
+
+flux completion zsh >| "$ZSH_CACHE_DIR/completions/_flux" &|