Add configurable runtime plugin to send_v2 so that it is usable (#2635)

## Motivation and Context
- `send_v2()` doesn't work for a very small number of reasons—allow it
one last final runtime_plugin so that we can use it to make an e2e
request

## Description
- update a few decorators to be deterministic
## Testing
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## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] 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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Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>