Allow extension name, description and author name to be translated

Hi guys,

In ACP >> Customise, extension name, description and author role are not translated.
It will be great if phpBB allow extensions author to use lang keys to translate this three (and maybe more) items.

suggested code to add in phpbb\extension\metadata_manager:

	/**

	 * Returns the language key if exists

	 *

	 * @return string

	 */

	public function get_metadata_language_key($lang_key)

	{

		if (isset($this->user->lang[$lang_key]))

		{

			return $this->user->lang[$lang_key];

		}

		return $lang_key;

	}