* and stop setting default values in python operator code, so defaults can be set at the server side.

This issue is just for the PR and questions from maintainers. 
airflow/providers/yandex/hooks/yandex.py
service_account_json_path = self._get_field('service_account_json_path', False)
        service_account_json = self._get_field('service_account_json', False)
        oauth_token = self._get_field('oauth', False)
        if not (service_account_json or oauth_token or service_account_json_path):
            raise AirflowException(
                'No credentials are found in connection. Specify either service account '
                + 'authentication JSON or user OAuth token in Yandex.Cloud connection'
            )
        if service_account_json_path:
            with open(service_account_json_path) as infile:
                service_account_json = infile.read()
        if service_account_json:
            service_account_key = json.loads(service_account_json)
            return {'service_account_key': service_account_key}
        else:
            return {'token': oauth_token}
    def _get_field(self, field_name: str, default: Any = None) -> Any:
        """Fetches a field from extras, and returns it."""
        long_f = f'extra__yandexcloud__{field_name}'
        if hasattr(self, 'extras') and long_f in self.extras:
            return self.extras[long_f]
        else:
            return default