Added method to created Stateful clients ( #268). (#603)

Co-authored-by: Daniel Rodriguez <daniel.rodriguez@elca.ch>

src/main/java/net/schmizz/sshj/SSHClient.java

@@ -722,6 +722,19 @@ public class SSHClient
         return new SFTPClient(new SFTPEngine(this).init());
     }
 
+    /**
+     * Stateful FTP client is required in order to connect to Serv-U FTP servers.
+     * @return Instantiated {@link SFTPClient} implementation.
+     *
+     * @throws IOException if there is an error starting the {@code sftp} subsystem
+     */
+    public SFTPClient newStatefulSFTPClient()
+            throws IOException {
+        checkConnected();
+        checkAuthenticated();
+        return new StatefulSFTPClient(new SFTPEngine(this).init());
+    }
+
     /**
      * Does key re-exchange.
      *