Request/Response type changes

[jdisanti]

In the previous release, the HTTP request type was changed. This release moves the request type added by that release into aws_smithy_runtime_api::http, and also adds a new response type with similar changes:

• Headers are restricted to valid UTF-8 strings. This means that returned values are strings, so conversion into bytes is no longer necessary
• If you need to produce an http = 0.2 Response from the Smithy Response, use into_http02x()
• A http::Response can be converted to a Smithy Response using its TryFrom implementation

In addition:

• The IMDS ErrorResponse::response now returns HttpResponse instead of http::Response<SdkBody>.
• The RequestId trait is no longer implemented for http::Response<B> or http::HeaderMap, but rather, for HttpResponse and Headers.
• The response type in the InterceptorContext is now HttpResponse instead of http::Response<SdkBody>.