Add support for reasoning_effort and extra_body parameters to ChatCompletionRequest

[TanKimzeg]

• reasoning_effort: Constrains inference effort for reasoning-capable models
• extra_body: Enables passing arbitrary JSON properties not yet typed in the SDK, providing forward compatibility with new API parameters without requiring SDK updates.

Usage:

// Reasoning effort
let req = ChatCompletion::builder("o3-mini", messages)
    .reasoning_effort("high")
    .build()?;

// Extra body
let mut extra = HashMap::new();
extra.insert("custom_param".into(), json!("custom"));
let req = ChatCompletion::builder("gpt-4o", messages)
    .extra_body(extra)
    .build()?;