Groovy rest client without libraries with basic authentication

Hello I need to connect to an url using a basic authentication with Groovy, I would prefer to achieve this without using any libraries, because in our project we have scan for vulnerabilities and some times we faced problems when we include a new library. I was following the next examples:

 def connection = new URL( "https://jsonplaceholder.typicode.com/posts")
        .openConnection() as HttpURLConnection

    // set some headers
    connection.setRequestProperty( 'User-Agent', 'groovy-2.4.4' )
    connection.setRequestProperty( 'Accept', 'application/json' )

    // get the response code - automatically sends the request
    println connection.responseCode + ": " + connection.inputStream.text

It worked but in my case I need to connect with a basic authentication (username and password). I haven't seen examples related.

Any ideas?

Thanks!