Add the 'extra_body' parameter

Question

### What feature would you like to see?

In the OpenAI Python SDK, there is a parameter called `extra_body`, which is useful for some third-party models and service provider APIs. For example, DeepSeek-V3.1 can enable deep thinking through:
```
extra_body = {
    "thinking": {"type": "enabled"},
}
```
It is hoped that `codex` can also support this parameter.

### Are you interested in implementing this feature?

_No response_

### Additional information

_No response_