feat(metadata): ✨ add support for explicit track metadata

* Added `explicit` attribute to the `Metadata` class.
* Updated the constructor to initialize `explicit` with a default value of `False`.
* Modified the metadata saving process to include explicit rating in tags.

tidal_dl_ng/download.py

@@ -1252,6 +1252,7 @@ class Download:
             url_share=track.share_url if track.share_url and self.settings.data.metadata_write_url else "",
             replay_gain_write=self.settings.data.metadata_replay_gain,
             upc=track.album.upc if track.album and track.album.upc else "",
+            explicit=track.explicit if hasattr(track, "explicit") else False,
         )
 
         m.save()