Refactor middleware to use new `operation::Response` instead of `http::Response<SdkBody>` (#635)

* Refactor Smithy service tower to include a property bag with the response

* Add doc comments and rename functions on operation Request/Response

* Fix codegen

* Update CHANGELOG

* Attach PR number to CHANGELOG

* Fix test compile error

* CR feedback

* Fix AWS runtime tests

* Fix doc comment link

* Fix API gateway customization

* Fix AWS integration tests and update design doc

* Make it possible to run IAM test outside CI and fix it