ebs + unsignedPayloadTrait support (#567)

* ebs + unsignedPayloadTrait support

1. Add support for the "unsignedPayoad" Smithy trait. When an operation is targetted with this trait, rather than signing the body of the operation, we will sign the literal string `Unsigned-Payload`.

This is the same behavior that will occur if the body is streaming and not directly signable.

2. Add an EBS example which utilizes this new signing behavior.

* Back out debugging change

* remove unused mut

* remove unused import

* fix clippy error

* Add protocol test for EBS

* Fix gradle build