feat: Add support for OpenAI's Structured Outputs in openai_dart

[davidmigloz]

• https://openai.com/index/introducing-structured-outputs-in-the-api
• https://platform.openai.com/docs/guides/structured-outputs

Example:

final res = await client.createChatCompletion(
  request: CreateChatCompletionRequest(
    model: ChatCompletionModel.model(
      ChatCompletionModels.gpt4oMini,
    ),
    messages: [
      ChatCompletionMessage.system(
        content:
            'You are a helpful assistant. That extracts names from text.',
      ),
      ChatCompletionMessage.user(
        content: ChatCompletionUserMessageContent.string(
          'John, Mary, and Peter.',
        ),
      ),
    ],
    temperature: 0,
    responseFormat: ResponseFormat.jsonSchema(
      jsonSchema: JsonSchemaObject(
        name: 'Names',
        description: 'A list of names',
        strict: true,
        schema: {
          'type': 'object',
          'properties': {
            'names': {
              'type': 'array',
              'items': {
                'type': 'string',
              },
            },
          },
          'additionalProperties': false,
          'required': ['names'],
        },
      ),
    ),
  ),
);
// {"names":["John","Mary","Peter"]}

[davidmigloz]

Reverted to split the PR in two