createMultiSpeakerRequest method
Create a multi-speaker TTS request
Implementation
GoogleTTSRequest createMultiSpeakerRequest({
  required String text,
  required Map<String, String> speakerVoices,
  String? model,
}) {
  final speakers = speakerVoices.entries
      .map((entry) => GoogleSpeakerVoiceConfig(
            speaker: entry.key,
            voiceConfig: GoogleVoiceConfig.prebuilt(entry.value),
          ))
      .toList();
  return GoogleTTSRequest.multiSpeaker(
    text: text,
    speakers: speakers,
    model: model ?? defaultTTSModel,
  );
}