feat: Support streaming audio responses in chat completions in openai_dart

[davidmigloz]

const request = CreateChatCompletionRequest(
  model: ChatCompletionModel.model(
    ChatCompletionModels.gpt4oAudioPreview,
  ),
  modalities: [
    ChatCompletionModality.text,
    ChatCompletionModality.audio,
  ],
  audio: ChatCompletionAudioOptions(
    voice: ChatCompletionAudioVoice.alloy,
    format: ChatCompletionAudioFormat.pcm16,
  ),
  messages: [
    ChatCompletionMessage.user(
      content: ChatCompletionUserMessageContent.string(
        'Is a golden retriever a good family dog?',
      ),
    ),
  ],
);
var chunks = 0;
var transcript = '';
var data = '';
final stream = client.createChatCompletionStream(request: request);
await for (final res in stream) {
  expect(res.choices, hasLength(1));
  final choice = res.choices.first;
  transcript += choice.delta.audio?.transcript ?? '';
  data += choice.delta.audio?.data ?? '';
  chunks++;
}
expect(chunks, greaterThan(1));
expect(transcript, contains('golden'));
expect(data, isNotEmpty);