Function connect 

pub async fn connect(
    stream: BoxStream,
    target: &Target,
    username: Option<&str>,
    password: Option<&str>,
) -> Result<BoxStream>

Perform an HTTP CONNECT tunnel handshake on an already-connected stream.

If username and password are provided, a Proxy-Authorization: Basic header is included.

Errors

Returns an error if the proxy responds with a non-2xx status code or the response headers cannot be parsed.