feat(argocd): add completion plugin

Co-authored-by: Sumudu Lansakara <sumudu.lansakara@ppro.com>

Closes #9900

plugins/argocd/README.md

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+# Argo CD plugin
+
+This plugin adds completion for the [Argo CD](https://argoproj.github.io/cd/) CLI.
+
+To use it, add `argocd` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... argocd)
+```
+
+This plugin does not add any aliases.
+
+## Cache
+
+This plugin caches the completion script and is automatically updated asynchronously when the plugin is
+loaded, which is usually when you start up a new terminal emulator.
+
+The cache is stored at:
+
+- `$ZSH_CACHE/completions/_argocd` completions script

plugins/argocd/gh.plugin.zsh

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