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.
I can't think of when a header would have a list of values, and one of them would be null.  I'd also use the for( Object role: roles) syntatic sugar, but that's just me.  You're call.