Function verify_connection 

async fn verify_connection(
    client: &Client,
    ip: &str,
    port: &str,
    protocol: &str,
    username: &str,
    password: &str,
) -> Result<(), Box<dyn Error>>

With the users specified AdGuard details, verify the connection. Returns Err on a failed connection (so the caller can retry); exits on rejected auth or an unsupported version, which retrying wouldn’t fix.