feat: add YouTube subtitle download & embed support (Fixes #288) (#1430)

* feat(youtube): add support for video captions in youtubeDownload

* feat(youtube): filter subtitles based on Items option in Extract method

* feat(youtube): display available caption languages in printInfo

* feat(youtube): add option to embed subtitles into video during download

* refactor(downloader): use utils.ConvertXmlToSrt and remove redundant function

* test(utils): add unit test for ConvertXMLToSRT function

* feat(readme): update subtitle download options and embed feature for YouTube

* refactor(downloader): rename ConvertXmlToSrt to ConvertXMLFileToSRT and update implementation

* refactor(ffmpeg): enhance subtitle handling with ISO 639-2 mapping and codec selection

* refactor(utils): simplify XML unmarshalling in ConvertXMLToSRT function