This adds an api endpoint `/files` to PRs that allows to get a list of changed files.
built upon #18228, reviews there are included
closes https://github.com/go-gitea/gitea/issues/654
Co-authored-by: Anton Bracke <anton@ju60.de>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>