Re-export `HttpRequest` and `HttpResponse` in client crates (#3762)

## Motivation and Context
https://github.com/smithy-lang/smithy-rs/issues/3591

## Description
In early days of smithy-rs, we used to re-export
[Request](https://docs.rs/aws-sdk-s3/0.25.0/aws_sdk_s3/client/customize/struct.Request.html)
and
[Response](https://docs.rs/aws-sdk-s3/0.25.0/aws_sdk_s3/client/customize/struct.Request.html)
types. When we overhauled the underlying smithy runtime from middleware
to orchestrator, we did not re-export the corresponding types
[HttpRequest](https://docs.rs/aws-smithy-runtime-api/latest/aws_smithy_runtime_api/client/orchestrator/type.HttpRequest.html)
and
[HttpResponse](https://docs.rs/aws-smithy-runtime-api/latest/aws_smithy_runtime_api/client/orchestrator/type.HttpResponse.html)
to client crates.

This PR will re-export them in `crate::config::http`.

## Testing
Added a new test file for `ClientRuntimeTypesReExportGenerator.kt` that
verifies re-exports.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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