Implement video-length filter using ffprobe

[Skaronator]

This implements two new filter options to filter videos based on their actual length before downloading them.

This fixes #4928

One interesting issue I noticed is that Motion JPEG exist. This is a video format that often only contains a single frame. Hence, I implemented the minimum frame logic.

[shelvacu]

I see a few problems with this approach. The request sent by ffprobe will not have any of the headers (most importantly cookies), the retry logic is missing, and this will 'double up' every request.

The proper (much more annoying to implement) way to do this would be to include ffmpeg as a library, and then do the requests in python and pass the data to the ffmpeg library.

I think it might make sense to include this nearly as-is (it still does something useful), but I think it should be marked with warnings in the documentation (maybe even rename the options something like experimental-min-length) and log a warning if headers["Cookies"] is set

[Skaronator]

Thanks for the feedback. I'll look into it

[Skaronator]

@shelvacu do you have an example link/page that explicitly requires specific headers? Thanks

[shelvacu]

Every site that requires a login will have to send a Cookie header. Other than that I'm not sure. I would suspect some sites insist on a Referer header but I'm not sure how to check which extractors add that.

[Skaronator]

I've implemented the same retry mechanism as the standard HTTP downloader. The code has been moved to a separate file for better readability, and I've also added the header logic.

Please feel free to test it out, and any feedback would be greatly appreciated, @shelvacu @mikf

[shelvacu]

Looks good, I hope this gets merged.