getSubtitlePlaylist  
  suspend fun getSubtitlePlaylist(itemId: <Error class: unknown class>, index: Int, mediaSourceId: String, segmentLength: Int): Response<ByteArray>
Gets an HLS subtitle playlist.
Parameters
itemId 
The item id.
index
The subtitle stream index.
mediaSourceId  
The media source id.
segmentLength 
The subtitle segment length.