Support write_image_jpeg with async

[edgarriba]

Add support for writing images with async that can be useful for i/o bound tasks in robotics and avoid people creating their threadpool.

requested by @haixuanTao for dora-rs

Current impl:

pub fn write_image_jpeg(file_path: &str, image: PyImage) -> PyResult<()> {
    let image = Image::from_pyimage(image)
        .map_err(|e| PyErr::new::<pyo3::exceptions::PyException, _>(format!("{}", e)))?;
    F::write_image_jpeg(file_path, &image)
        .map_err(|e| PyErr::new::<pyo3::exceptions::PyException, _>(format!("{}", e)))?;
    Ok(())
}

proposal with async

pub fn write_image_jpeg(file_path: &str, image: PyImage) -> PyResult<()> {
    let image = Image::from_pyimage(image)
        .map_err(|e| PyErr::new::<pyo3::exceptions::PyException, _>(format!("{}", e)))?;
    use std::thread;
    thread::spawn(move || {
      F::write_image_jpeg(file_path, &image)
          .map_err(|e| PyErr::new::<pyo3::exceptions::PyException, _>(format!("{}", e)))?;
    });

    Ok(())
}

Discussed to return a join Handle so that people can query state pending, failure or success