[ie] Add new fields with proper support for multiple values

[llistochek]

Description of your pull request and other information

Partially fixes #3944 (We need to add support in extractors themselves).

Changes:

• [InfoExtractor] Added new fields:
  • artist_list
  • composer_list
  • genre_list
  • album_artist_list
• [InfoExtractor] Reflected changes in documentation
• [YoutubeDL] Added new function fix_deprecated_fields, which converts deprecated fields into their new counterpart , splitting them by comma.
• [FFMpegMetadataPP] Replaced all usages of deprecated fields with new, added support for composer
• [FFMpegMetadataPP] Slightly changed add function inside _get_metadata_opts to support lists
• Added new post processor MutagenMetadataPP. It runs just after FFMpegMetadataPP and overrides ffmpeg-written metadata with properly formatted values. It writes only new (*_list) fields.

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[pukkandan]

• Added new post processor MutagenMetadataPP. It runs just after FFMpegMetadataPP and overrides ffmpeg-written metadata with properly formatted values. It writes only new (*_list) fields.

Could you split it into a new PR? I can merge the rest asap, but this part needs further discussion. Your impl is not matching what we discussed previously

• artist_list
• composer_list
• genre_list
• album_artist_list

Let's go with artists, composers etc. And update the fields in readme too https://github.com/yt-dlp/yt-dlp#outtmpl-postprocess-note