[ie/vk] Improve format extraction

[seproDev]

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Sets the extension for http videos to mp4. While we can't be 100% sure that this will be the case for every video, I still think this is vastly preferable to unknown_video. Currently, all videos I checked are served as MP4s.
I've also added dash format extraction. This video contains vp9 and av1 dash formats.

Fixes #5675

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