docs: public static String expand(ServerConfiguration config, String value) {
          if (value == null)
              return null;
  
          if (!value.startsWith("${") || !value.endsWith("}"))
              return value;
  
          String variableName = value.substring(2, value.length() - 1);
          Variable var = config.getVariables().getBy("name", variableName);
          return var.getValue();
      }