Added support for per-request authentication to Jsoup.connect

[jhy]

Uses the multi-version support so that in Java versions that support it (9+), an authenticator is set via java.net.HttpURLConnection.setAuthenticator().

On Java 8, we set the system-wide default authenticator, and use ThreadLocals to enable per-request authenticators.

Also adds tests for HTTP and HTTPS server and proxy basic authentication.

Example use:

 Connection session = Jsoup.newSession()
     .proxy("proxy.example.com", 8080)
     .auth(auth -> {
         if (auth.isServer()) { // provide credentials for the request url
             Validate.isTrue(auth.url().getHost().equals("example.com"));
             // check that we're sending credentials were we expect, and not redirected out
             return auth.credentials("username", "password");
         } else { // auth.isProxy()
             return auth.credentials("proxy-user", "proxy-password");
         }
     });