Fix private-in-public error in rustc before 1.74.0

[nerdile]

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

• 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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[ysaito1001]

LGTM, we probably just need to bump a patch version of aws-smithy-runtime in its Cargo.toml

[nerdile]

@rcoh :

• PR bot and canary cannot be invoked from a forked repository. Ask a maintainer to manually invoke them using your PR.
• check-sdk-codegen-unit-tests failing to find recently published tokio-macros crate
  Anything I can do to resolve these?

[nerdile]

LGTM, we probably just need to bump a patch version of aws-smithy-runtime in its Cargo.toml

Thanks @ysaito1001 , is this something I should address or will the team take care of it?

[ysaito1001]

We'll handle the rest and get it merged because the rest is a small chore. Thanks for the PR!

Passed canary run