feat(structured outputs): add support for accessing raw responses

[RobertCraigie]

You can now access the raw response for the structured outputs .parse() helper.

response = client.beta.chat.completions.with_raw_response.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=MathResponse,
)
print(response.headers.get("x-my-header"))

completion = response.parse()

message = completion.choices[0].message
if message.parsed:
    rich.print(message.parsed.steps)

    print("answer: ", message.parsed.final_answer)
else:
    print(message.refusal)