[ticket/12685] New CLI container for load extensions

https://tracker.phpbb.com/browse/PHPBB3-12685

PHPBB3-12685
We forgot to setup the class loaders for the extensions.
So if the safe-mode is not enabled we need to add these lines: (like in common.php)
```
$phpbb_class_loader_ext = new \phpbb\class_loader('\\', "{$phpbb_root_path}ext/", $phpEx);
$phpbb_class_loader_ext->register();
phpbb_load_extensions_autoloaders($phpbb_root_path);
```