feat(mongo-atlas): add completion for atlas (#11349)

plugins/mongo-atlas/README.md

+# MongoDB Atlas plugin
+
+This plugin adds completion for [Atlas](https://www.mongodb.com/docs/atlas/cli/stable/) a command line interface built specifically for 
+MongoDB Atlas.
+
+To use it, add `mongo-atlas` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... mongo-atlas)
+```

plugins/mongo-atlas/mongo-atlas.plugin.zsh

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