Refactor middleware to use new operation::Response instead of http::Response<SdkBody>

[jdisanti]

Motivation and Context

This is progress towards #121.

This change is needed to be able to support Event Streams, since we need to track the original request's signature through to the first streaming event, as event frames are recursively signed with the previous event's or top-level request's signature.

Description

• Creates operation::Response which attaches a PropertyBag to an http::Response<SdkBody>.
• Updates ParseHttpResponse's parse_unloaded to take an operation::Response instead of an http::Response<SdkBody> so that it has access to the property bag. This isn't needed for the signature propagation, but may be needed to track the Body::channel that's created when attaching the input event stream to the HTTP request.
• Renames config/request methods on operation::Request to properties/http so that they're consistent with operation::Response, and so that config doesn't get confused with the top-level generated service Config.
• Adds doc comments to operation::Request

Testing

• cargo test, cargo clippy on rust-runtime and aws/rust-runtime
• g codegen:test && g codegen-test:test && g aws:sdk:assemble
• Manually built and ran the S3 ListBuckets example

Checklist

• I have updated the CHANGELOG

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rcoh]

LGTM except doc comment