Allow to apply a single effect to a single track

[kuamanet]

Example of usage

try {
      // AAC example: https://dl.espressif.com/dl/audio/ff-16b-2c-44100hz.aac
      const baseAudioSource = AudioSource.uri(Uri.parse("asset:///audio/nature.mp3"));
      const audioSourceWithEffects =
          AudioSource.uri(Uri.parse("asset:///audio/nature.mp3")).pipeline(AudioPipeline(
        audioEffects: [
          Equalizer(), // iOS - Android cross platform equalizer
          Delay(), // Delay audio effect
          Reverb(), // Reverb audio effect
        ],
      ));

      await _player.setAudioSource(
        ConcatenatingAudioSource(
          children: [
            baseAudioSource,
            audioSourceWithEffects,
          ],
        ),
      );
    } catch (e) {
      print("Error loading audio source: $e");
    }