Use file_system service for creating absolute pathnames

There seems to be an issue when setting up public and private keys with stream URIs on admin/config/people/simple_oauth configuration form. The problem is in the usage of PHP native realpath function, which does not support stream wrappers. These calls should be replaced with the Drupal file_system realpath() method, which checks if URI is a stream and converts it to an absolute path.

Currently there are 2 places where PHP realpath() function is used to generate absolute pathnames.

In Oauth2GrantManager

$server = new AuthorizationServer(
      $this->clientRepository,
      $this->accessTokenRepository,
      $this->scopeRepository,
      realpath($this->privateKeyPath),
      Core::ourSubstr($salt, 0, 32),
      $this->responseType
    );

and in ResourceServer

try {
      $public_key = $config_factory->get('simple_oauth.settings')->get('public_key');
      $public_key_real = realpath($public_key);
      if ($public_key && $public_key_real) {
        $this->subject = new LeageResourceServer(
          $access_token_repository,
          $public_key_real
        );
      }
    }