feat: add support for graceful exits

[rupansh]

This adds a new flag --graceful-exit, which sends SIGTERM signal to the server process on unix hosts and Ctrl+Break on windows.

Note that for windows, I only checked the compilation. so testing on windows would be greatly appreciated.

[rupansh]

I fired up a windows VM for testing and found that CTRL+C event doesn't work. Applications must handle ctrl+break on windows for graceful exits when spawn via cargo leptos

here's the termination future I passed to with_graceful_shutdown in axum:

    let terminate = {
        use tokio::signal;

        let ctrl_c = async {
            signal::ctrl_c()
                .await
                .expect("failed to install Ctrl+C handler");
        };

        #[cfg(unix)]
        let terminate = async {
            signal::unix::signal(signal::unix::SignalKind::terminate())
                .expect("failed to install signal handler")
                .recv()
                .await;
        };

        #[cfg(windows)]
        let terminate = async {
             signal::windows::ctrl_break()
                 .expect("failed to install Ctrl+Break handler")
		     .recv()
                 .await;
	  };

        async {
            tokio::select! {
                _ = ctrl_c => {},
                _ = terminate => {},
            }
            logging::log!("stopping...");
        }
    };
    ```