Http updates (#3059)

## Motivation and Context
- [ ] Upgrade guidance
- [x] changelog

To allow seamless adoption of the incoming `http` crate, we're updating
our client crates to use an intermediate type.

## Description

Update _client_ code to use `HttpRequest`. Some internal code still uses
`http::Request` but those usages are limited.

## Testing
CI

## 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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