RequestHandlerRetryAdvice wraps endpoint exception in a MessagingException, harder to do exception classifcation based retry

When adding retry advice to an endpoint you probably want to retry for some exceptions and fail immediately for other ones.

This is supported easily by RetryTemplate with the default BasicRetryPolicy which uses an exception classifier.

However, the RetryCallback constructed by RequestHandlerRetryAdvice wraps the actual business exception thrown by the endpoint with a MessagingException, making the default configuration-only classification impossible.