Add more client re-exports (#2437)

* Add more client re-exports

This commit adds more client re-exports proposed in
https://github.com/awslabs/smithy-rs/issues/1759

.
Specifically, it re-exports the following types:
- `aws_smithy_http::body::SdkBody`
- `aws_smithy_http::byte_stream::error::Error`
- `aws_smithy_http::operation::{Request, Response}`

* Update CHANGELOG.next.toml

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Co-authored-by: Yuki Saito <awsaito@amazon.com>