Fix private-in-public error in rustc before 1.74.0 (#3675)

## Motivation and Context
Rustc 1.74.0 removed the private-in-public check, which raises a
compile-time error if a member of a struct has a type which is less
visible than the parent struct. Fixing this private-in-public issue
enables aws-smithy-runtime to build in projects using rustc older than
1.74.0.

## Description
Make UploadThroughputCheckFuture pub(crate) because it is needed in
MaybeUploadThroughputCheckFuture which is pub(crate).

## Testing
Verified that crate now compiles using rustc 1.72.1.

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