networking: add ability to manually add wifi networks

[soramanew]

Currently, you can only connect to a wireless network that is exposed in the NetworkDevice::networks property. This PR adds the ability to connect to networks that do not expose their SSID, or that don't show up in the networks list for whatever reason.

This is done via a new WifiDevice::addNetwork method, which takes the SSID, security type and options, and whether or not the network is hidden.

Example usage:

Item {
    required property WifiDevice wifi
    
    Component.onCompleted: {
        wifi.addNetwork("some_ssid", WifiSecurityType.Wpa2Psk, {
            psk: "some_password"
        });
    }
}