[Feature Request] Get more metadata with ffprobe

[kr4ssi]

• [v] I'm reporting a feature request
• [v] I've verified that I'm running yt-dlp version 2021.08.02
• [v] I've searched the bugtracker for similar feature requests including closed ones

Another idea I had a while ago is to include the duration in the info-json even if it's not revealed by the provider, by ffprobing the file. Preferably with less requests as possible, like when downloading a file (and dumping the info-json to a file before the download completed) there shouldn't be two requests to it.