feat(k9s): add completion plugin for `k9s` (#12691)

Co-authored-by: Marc Cornellà <marc@mcornella.com>

plugins/k9s/README.md

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+# k9s plugin
+
+This plugin adds completion support for the [k9s](https://k9scli.io).
+
+To use it, add `k9s` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... k9s)
+```

plugins/k9s/k9s.plugin.zsh

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+if (( ! $+commands[k9s] )); then
+  return
+fi
+
+# If the completion file does not exist, fake it and load it
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_k9s" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _k9s
+  _comps[k9s]=_k9s
+fi
+
+# and then generate it in the background. On first completion,
+# the actual completion file will be loaded.
+k9s completion zsh >| "$ZSH_CACHE_DIR/completions/_k9s" &|