Function send_command 

pub fn send_command<App, Req, Res, H>(
    app: App,
    command: &Req,
    handler: Option<H>,
) -> Result<()>
where
    App: ToString,
    Req: Serialize,
    Res: for<'de> Deserialize<'de> + Debug,
    H: Fn(Res) + Send + 'static,