Allow streaming of raw PGS subtitles without transcoding

[Arcus92]

Changes This PR adds support for streaming raw PGS subtitles (graphical subtitles) if requested by the client. PGS subtitles can be loaded from external .sub / .sup files or extracted from an .mkv container. This removed the need for video transcoding just for the PGS subtitle.

Related Pull Request: https://github.com/jellyfin/jellyfin-web/pull/5688

Discussion points:

• Extracting subtitles from large .mkv can take multiple minutes for ffmpeg to process the whole file. This is only be done once per media file. The subtitles are cached afterwards.
• PGS subtitles are large (multiple megabytes) and may take a while to load.