Make `CustomizableOperation` `Send` and `Sync` (#2951)

## Motivation and Context
Fixes https://github.com/awslabs/smithy-rs/issues/2944

## Description
`CustomizableOperation` in the last two releases
([release-2023-08-01](https://github.com/awslabs/smithy-rs/releases/tag/release-2023-08-01)
and
[release-2023-08-22](https://github.com/awslabs/smithy-rs/releases/tag/release-2023-08-22)

)
broke backward-compatibility with respect to its auto traits.
Specifically, it ceased to be `Send` and `Sync` because the struct
contained a boxed trait object. This PR fix that issue, making
`CustomizableOperation` `Send` and `Sync` again.

## Testing
- Added a Kotlin unit test to verify `CustomizableOperation` is `Send`
and `Sync`.

## 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

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

Co-authored-by: ysaito1001 <awsaito@amazon.com>