Split test-util feature to allow test-util to be utilized in WASM (#2873)

## Motivation and Context
- #2087 

test-util pulled in Hyper but that's not strictly necessary

## Description
Split out wiremock as it's own features and do some other cleanup.

## 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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