feat: Add Song radio option in tracklist.

At this moment this feature is only available on the Subsonic media provider. Using a
Jellyfin server the "Play song radio" option will be disabled.

backend/mediaprovider/mediaprovider.go

@@ -200,6 +200,10 @@ type SupportsSharing interface {
 	CanShareArtists() bool
 }
 
+type SupportsSongRadio interface {
+	GetSongRadio(trackID string, count int) ([]*Track, error)
+}
+
 type LyricsProvider interface {
 	GetLyrics(track *Track) (*Lyrics, error)
 }

backend/mediaprovider/subsonic/subsonicmediaprovider.go

@@ -543,3 +543,11 @@ func fillPlaylist(pl *subsonic.Playlist, playlist *mediaprovider.Playlist) {
 	playlist.TrackCount = pl.SongCount
 	playlist.Duration = pl.Duration
 }
+
+func (s *subsonicMediaProvider) GetSongRadio(trackID string, count int) ([]*mediaprovider.Track, error) {
+	tr, err := s.client.GetSimilarSongs(trackID, map[string]string{"count": strconv.Itoa(count)})
+	if err != nil {
+		return nil, err
+	}
+	return sharedutil.MapSlice(tr, toTrack), nil
+}