JCLOUDS-480 start supporting version 4 signatures for aws-ec2.

This is a work-in-progress, but I've verified through manual testing that it works in frankfurt. The code is a variant of something I've created in the past https://github.com/Netflix/feign/blob/master/sax/src/test/java/feign/sax/examples/AWSSignatureVersion4.java

As ec2 has some really old unit tests, there's still some grunt work needed to complete this change.
changes here are just indenting due to bumping the impl out to its own class.