D-17434: implements ETag postfix tag based on some header values in the HTTP request

Created a new class called HeaderValueEntityTagProcessor that knows how to append a string in ETag based on some HTTP request header values.
Things would not necessarily error out. You just won't get the right ETag. Or we would not be getting the right atom:updated timestamp (that's what FeedPagingProcessor() does). This protects us for possible mis-configuration.

I think the interface is cleaner with that if statement. Each constructor is meant to be used for one purpose. If you don't care to override the list of AdapterResponseInterceptor, then use the other constructor. I can put some comments there if necessary.