[nbcolympics:stream] Fix extractor

PR: https://github.com/ytdl-org/youtube-dl/pull/29688
Closes: #617, https://github.com/ytdl-org/youtube-dl/issues/29665

* Livestreams are untested
* If using ffmpeg as downloader, v4.3+ is needed since `-http_seekable` option is necessary
* Instead of making a seperate key for each arg that needs to be passed to ffmpeg, I made `_ffmpeg_args`
* This deprecates `_seekable`, but the option is kept for compatibility

Authored by: nchilada, pukkandan

yt_dlp/downloader/external.py

@@ -382,6 +382,9 @@ class FFmpegFD(ExternalFD):
         if not self.params.get('verbose'):
             args += ['-hide_banner']
 
+        args += info_dict.get('_ffmpeg_args', [])
+
+        # This option exists only for compatibility. Extractors should use `_ffmpeg_args` instead
         seekable = info_dict.get('_seekable')
         if seekable is not None:
             # setting -seekable prevents ffmpeg from guessing if the server