Add OpenAI support for structured outputs

[trevbeers-thrive]

If response_format is passed as a kwarg, use client.beta.chat.completions.parse instead of client.beta.chat.completions.create.

Usage example:

import aisuite as ai
from pydantic import BaseModel

client = ai.Client()

class Joke(BaseModel):
    joke: str

messages = [
    {"role": "system", "content": "Respond in Pirate English."},
    {"role": "user", "content": "Tell me a joke."},
]

response = client.chat.completions.create(
    model="openai:gpt-4o",
    messages=messages,
    response_format=Joke,
    temperature=0.75
)
print(response.choices[0].message.content)