Finish bufferred samples

[RomanLut]

AudioGenerator should let AudioOutput finish playing buffered samples.

Reproduction:

• ESP32, Create AudioOutputI2S with large number of buffers: AudioOutputI2S(port, 1, 80)
• play mp3 or WAV file
  => large part of sound file is not played

The common loop:

if (mp3->isRunning()) {
if (!mp3->loop()) mp3->stop();
} else {
Serial.printf("MP3 done\n");
delay(1000);
}

stops AudioOutput as soon as last sample is pushed to output. All buferred samples are discarded.

Solution:
After pushing last sample, AudioGenerator should call AudioOutput->finish() as long as it returns false.
While finishing, AudioGenerator should return true from AudioGenerator->IsRunning() and AudioGenerator->loop().

Implemented for AudioOutputI2S ( and AudioOutputNoDAC effectivelly ) , and AudioGeneratorMP3 and AudioGeneratorWAV.
Other classes work as before ( not affected ).

[earlephilhower]

Thanks for the contribution, but the code has diverged quite a bit and we have many merge conflicts now. Closing.