feat(gcx): add completion plugin for Grafana's gcx (#13950)

* feat(gcx): add completion plugin

* fix(gcx): write completion cache atomically

plugins/gcx/gcx.plugin.zsh

@@ -0,0 +1,18 @@
+# Autocompletion for the Grafana CLI (gcx).
+if (( ! $+commands[gcx] )); then
+  return
+fi
+
+# If the completion file doesn't exist yet, we need to autoload it and
+# bind it to `gcx`. Otherwise, compinit will have already done that.
+if [[ ! -f "$ZSH_CACHE_DIR/completions/_gcx" ]]; then
+  typeset -g -A _comps
+  autoload -Uz _gcx
+  _comps[gcx]=_gcx
+fi
+
+zmodload -F zsh/files b:zf_mv
+() {
+  local TMPPREFIX="$ZSH_CACHE_DIR/completions/_gcx"
+  zf_mv -f -- =( gcx completion zsh ) "$TMPPREFIX"
+} &|