Add video support

[lhoestq]

videos in /first-rows and /rows will be like

{"src": "https://huggingface.co/datasets/username/dataset_name/resolve/commit/hash/path/to/video.mp4"}

when possible, otherwise the back-ends store videos in the assets on S3 as usual (e.g. if they come from parquet or webdataset data or zipped data)

TODO: support hf:// urls in URLPreparator

[severo]

I remember you mentioned at one point that some video datasets have one video, and each row refers to a range [start, end], which would be accessed as "media fragments" in HTML: https://developer.mozilla.org/en-US/docs/Web/Media/Audio_and_video_delivery#specifying_playback_range. Is it still a thing, or do we just have one row = one video?

[lhoestq]

one row = one video (we'll see later if we need to support other cases)