feat(structured outputs): support accessing raw responses

[RobertCraigie]

closes #1057

You can now do this:

  const { data: completion, response } = await client.beta.chat.completions
    .parse({
      model: 'gpt-4o-2024-08-06',
      messages: [
        { role: 'system', content: 'You are a helpful math tutor.' },
        { role: 'user', content: 'solve 8x + 31 = 2' },
      ],
      response_format: zodResponseFormat(MathResponse, 'math_response'),
    })
    .withResponse();