Remove the public HTTP dependency from aws-sigv4 (#2921)

## Motivation and Context
Removes the public http dependency from the aws-sigv4 crate to avoid
compatibility issues with http = 1.0 and to support the http refactor

## Description
- Changes `SignableRequest::new` to remove the direct exposure of HTTP
types
- Assorted test refactorings as a result
- Update calling code

## Testing
IT/UT

## Checklist
TODO: changelog
<!--- 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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