feat(skaffold): add completion (#11357)

plugins/skaffold/README.md

+# Skaffold plugin (Autocompletion)
+
+This plugin adds completion for [Skaffold](https://skaffold.dev)
+
+To use it, add `skaffold` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... skaffold)
+```

plugins/skaffold/skaffold.plugin.zsh

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