stream_curl: support offset/end_offset passed in AVDictionary

lavf's http demuxer exposes initial/final byte offsets as AVOptions.
Some demuxers, like lavf/hls.c assume it is always available, even for
custom IO. We need to use those values for correct demuxing.

Fixes: https://github.com/mpv-player/mpv/pull/17879#issuecomment-4468476976