List the objects for a bucket.

        :param str instance: A Yamcs instance name.
        :param str bucket_name: The name of the bucket.
        :param str prefix: If specified, only objects that start with this
                           prefix are listed.
        :param str delimiter: If specified, return only objects whose name
                              do not contain the delimiter after the prefix.
                              For the other objects, the response contains
                              (in the prefix response parameter) the name
                              truncated after the delimiter. Duplicates are
                              omitted.

def list_objects(self, instance, bucket_name, prefix=None, delimiter=None):
        """
        List the objects for a bucket.

        :param str instance: A Yamcs instance name.
        :param str bucket_name: The name of the bucket.
        :param str prefix: If specified, only objects that start with this
                           prefix are listed.
        :param str delimiter: If specified, return only objects whose name
                              do not contain the delimiter after the prefix.
                              For the other objects, the response contains
                              (in the prefix response parameter) the name
                              truncated after the delimiter. Duplicates are
                              omitted.
        """
        url = '/buckets/{}/{}'.format(instance, bucket_name)
        params = {}
        if prefix is not None:
            params['prefix'] = prefix
        if delimiter is not None:
            params['delimiter'] = delimiter
        response = self._client.get_proto(path=url, params=params)
        message = rest_pb2.ListObjectsResponse()
        message.ParseFromString(response.content)
        return ObjectListing(message, instance, bucket_name, self)