Import HAR file as recording

[MattiasBuelens]

This adds a streamrr import command, which allows you to import an HTTP Archive (.har) file as a recording. The HLS playlists and segments are extracted, rewritten and stored on disk in the same format as streamrr record.

This allows users to save a HAR file from a playback session in their web browser, rather than running streamrr record on the side. This can be useful if a stream requires complicated authentication that can only be done in a real browser, or if the user has already received a HAR file from somewhere else.

To do:

• For livestreams, ensure we correctly advance to the next HLS playlist while importing.
• If segments are missing from the HAR file, mark them with #EXT-X-GAP in the rewritten playlist and skip them.