Allow rippers as audio providers

[tonarec]

Add Spytify to be used as a ripper to download tracks, but keep using SpotDL for metadata.

Use the following workflow:

• Delete the tracks we want to re-download
• Start from the URL of the Spotify playlist
• Get the spotdl file
• Let the sync() method computes the songs
• From now, we need to fake the downloader
  • Rippers will download songs into a specific folder (eg. Spot2Box/RippersOutputs)
  • Before processing a song (eg. search_and_download) we need to check if the songs exists in this folder (meaning rippers have downloaded tracks externally)
  • To force the downloader to use those files, but force metadata we need to move/replace the file in the outputs folder
  • Make sure the downloader see this track as "new" for metadata, instead of forcing metadata for all tracks (avoid update all MP3s each time)

Another workflow:

• Don't touch the spotdl process
• Rip tracks into Spot2Box/Rippers outputs
• Check from duplicates into outputs vs. rippers
• If present, apply metadata then replace the song

See this method from Spytify : public override string ToString()