Add an option to configure RSS fetch timeout

### Description

I'm using [RSSHub](https://github.com/DIYgod/RSSHub) for turning many websites to RSS feeds. When using with the RSS widget in Glance, I met a problem.

Some platforms, like `/bilibili/user/dynamic`, will take relatively long time (more than 20 seconds) to fetch all information and turn them into a feed. But before the response successfully returned, Glance will throw an error as "timed out". Reloading the page doesn't help (I guess the RSS feed response (error) is cached in Glance so it will not fetch again)

I wish there's some workaround to solve this problem. Maybe allowing configuring the fetch timeout?