code: public void addUrlSeed(String url, String externAuth, List<Pair<String, String>> extraHeaders) {
          string_string_pair_vector v = new string_string_pair_vector();
  
          for (Pair<String, String> p : extraHeaders) {
              v.push_back(p.to_string_string_pair());
          }
  
          ti.add_url_seed(url, externAuth, v);
      }
docs: Adds one url to the list of url seeds. Currently, the only transport protocol
  supported for the url is http.
  <p>
  The {@code externAuth} argument can be used for other authorization schemes than
  basic HTTP authorization. If set, it will override any username and password
  found in the URL itself. The string will be sent as the HTTP authorization header's
  value (without specifying "Basic").
  <p>
  The {@code extraHeaders} argument can be used to insert custom HTTP headers
  in the requests to a specific web seed.
  
  @param url
  @param externAuth
  @param extraHeaders