feat: create body extractor function

[Valerioageno]

Checklist

• I have read Contributing > Pull requests

Related issue

Fixes #538

Overview

Usage

#[derive(Deserialize)]
struct MyBody {
   pub field: u8
}

#[tuono_lib::api(POST)]
async fn my_post_request(req: Request) -> impl IntoResponse {
    let body = req.body::<MyBody>().unwrap();
}

• Finish to do the tests