v2.6.9

1. 增加下载本子封面图的函数以及download_cover插件
2. 修复client.after_init时advance_retry插件没有生效的问题(#472) (#482)

What's Changed

🏕 Features

• v2.6.9: 增加下载本子封面图的函数以及download_cover插 by @hect0x7 in #482

Full Changelog: v2.6.7...v2.6.9