Add identifier field to PendingPlaylist in PlayOutcome

Updated `PendingPlaylist` to include a new `identifier` field, enabling more precise playlist identification. Adjusted `SearchResultHandler` and `MusicRoutes` to use and expose this field, ensuring integration across the system and improving API consistency.

src/main/java/me/bobert/netneutrality/music/SearchResultHandler.java

@@ -66,7 +66,7 @@ public class SearchResultHandler extends AbstractAudioLoadResultHandler {
         List<Track> candidates = truncated ? tracks.subList(0, maxPlaylistSize) : tracks;
 
         String searchId = PendingSearchResults.put(guild.getId(), candidates);
-        future.complete(new PlayOutcome.PendingPlaylist(searchId, result.getInfo().getName(), candidates, truncated));
+        future.complete(new PlayOutcome.PendingPlaylist(searchId, identifier, result.getInfo().getName(), candidates, truncated));
     }
 
     @Override