Find usage on Client Certificates. Update `self.limits`.

def _find_usage_certs(self):
        """
        Find usage on Client Certificates. Update `self.limits`.
        """
        logger.debug('Finding usage for Client Certificates')
        cert_count = 0
        paginator = self.conn.get_paginator('get_client_certificates')
        for resp in paginator.paginate():
            cert_count += len(resp['items'])
        self.limits['Client certificates per account']._add_current_usage(
            cert_count, aws_type='AWS::ApiGateway::ClientCertificate'
        )