Add plugin for LXD autocomplete (#7457)

plugins/lxd/README.md

+# lxd
+
+This plugin provides completion for [lxd](https://linuxcontainers.org/lxd/), as well as aliases
+for frequent lxc commands.
+
+To use it add `lxd` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... lxd)

plugins/lxd/lxd.plugin.zsh

+_lxc_get_command_list () {
+    $_comp_command1 | sed "1,/Available Commands/d" | awk '/^[ \t]*[a-z]+/ { print $1 }'
+}
+
+_lxc_get_subcommand_list () {
+    $_comp_command1 ${words[2]} | sed "1,/Available Commands/d" | awk '/^[ \t]*[a-z]+/ { print $1 }'
+}
+
+_lxc () {
+  local curcontext="$curcontext" state line
+  typeset -A opt_args
+  _arguments \
+    '1: :->command'\
+    '*: :->args'
+
+  case $state in
+    command)
+      compadd $(_lxc_get_command_list)
+      ;;
+    *)
+        compadd $(_lxc_get_subcommand_list)
+      ;;
+  esac
+}
+
+compdef _lxc lxc