Enabled to AddConnection for connecting hidden SSID

[MoriKeisuke]

I add function of AddConnection in NetworkManager.Settings.
Because I want to connect to hidden SSID.

I had searched https://github.com/OpenAgricultureFoundation/python-wifi-connect flow, and found network-manager to need AddConnection function.
I built and to run wifi-connect included this branch's, and verified to connect to hidden SSID.

# from https://github.com/balena-io/wifi-connect/blob/master/src/network.rs
impl NetworkCommandHandler {
    // ...
    fn connect(&mut self, ssid: &str, identity: &str, passphrase: &str) -> Result<bool> {
        // ...
        if let Some(access_point) = find_access_point(&self.access_points, ssid) {
            // ...
        } else {
            let wifi_device = self.device.as_wifi_device().unwrap();
            // Use there!
            let _ = wifi_device.add_connection(ssid, &AccessPointCredentials::Wpa {passphrase: passphrase.to_string()});

            if let Ok(connections) = self.manager.get_connections() {
                for conn in connections {
                    if conn.settings().ssid.as_str().unwrap_or("SSID not found") == ssid {
                        match conn.activate() {
                            // same
                        }
                    }
                }
            }
        }
    }
    // ...
}

The AddConnection() method
https://developer.gnome.org/NetworkManager/stable/gdbus-org.freedesktop.NetworkManager.Settings.html#gdbus-method-org-freedesktop-NetworkManager-Settings.AddConnection