code: public MobileIDSession startLogin(String personalCode, String countryCode) {
      return startLogin(personalCode, countryCode, null);
    }
docs: Initiates the login session. Note: returned session already contains user's info, but the authenticity is not yet verified.
  
  @param personalCode user's personal code
  @param countryCode  two letter country code, eg EE
  @throws AuthenticationException is authentication unsuccessful
  @return MobileIDSession instance containing CHALLENGE ID that should be shown to the user