Allow connections through proxies by specifying an AWS ClientConfiguration bean

Hello.

You did a really great job by providing Spring integration support for SNS/SQS. 
I just played around with your solution in a small project and, in the first place, it works very well. Now I recognized that I have to connect to Amazon through a proxy and was not able to figure out how to do it with your library.

I made some small changes that allows an optional specification of a ClientConfiguration object which is used in the subsequent creation of AmazonSNSClient/AmazonSQSClient.

May you have a look into may changes please? Thanks!

Best regards,
André