private Request<?> presignRequest(Request<?> request, AWSCredentials credentials, String signingRegion) {
        AWS4Signer signer = createNewSignerWithRegion(signingRegion);
        signer.presignRequest(request, credentials, null);
        return request;
    }
Abstract pre-sign handler that follows the pre-signing scheme outlined in the 'RDS Presigned URL for Cross-Region Copying' SEP.

@param <T> The request type.
aws-java-sdk-neptune/src/main/java/com/amazonaws/services/neptune/PresignRequestHandler.java