Is your feature request related to a problem? Please describe.
Many websites use Google's FeedBurner to serve their public feeds. This allows Google to track you when you use the feed, and also makes it impossible to fetch full article content with FreshRSS.

Describe the solution you'd like
Publishers create FeedBurner feeds from normal RSS feeds. However, in many cases the original feed is only accessible with a user-agent of FeedBurner (https://support.google.com/feedburner/answer/78464). Adding an option to have FreshRSS request these feeds using a FeedBurner user agent would allow users to read the original version of these feeds in FreshRSS.