Function get_async 

pub fn get_async(
    key: &str,
    reason: &str,
    on_done: impl FnOnce(Result<Secret, KeychainError>) + Send + 'static,
)

The item under key, showing the OS’s verification dialog with reason when the item needs the person present, and answering exactly once, on the platform’s thread. An item at another level resolves without a dialog.