code: public AuthRequest login(String emailOrUsername, String password) {
          Asserts.assertNotNull(emailOrUsername, "email or username");
          Asserts.assertNotNull(password, "password");
  
          String url = baseUrl
                  .newBuilder()
                  .addPathSegment(PATH_OAUTH)
                  .addPathSegment(PATH_TOKEN)
                  .build()
                  .toString();
          TokenRequest request = new TokenRequest(client, url);
          request.addParameter(KEY_CLIENT_ID, clientId);
          request.addParameter(KEY_CLIENT_SECRET, clientSecret);
          request.addParameter(KEY_GRANT_TYPE, "password");
          request.addParameter(KEY_USERNAME, emailOrUsername);
          request.addParameter(KEY_PASSWORD, password);
          return request;
      }
docs: Creates a log in request using the 'Password' grant and the given credentials.
  i.e.:
  <pre>
  {@code
  AuthAPI auth = new AuthAPI("me.auth0.com", "B3c6RYhk1v9SbIJcRIOwu62gIUGsnze", "2679NfkaBn62e6w5E8zNEzjr-yWfkaBne");
  try {
  TokenHolder result = auth.login("me@auth0.com", "topsecret")
  .setScope("openid email nickname")
  .execute();
  } catch (Auth0Exception e) {
  //Something happened
  }
  }
  </pre>
  
  @param emailOrUsername the identity of the user.
  @param password        the password of the user.
  @return a Request to configure and execute.