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

----

_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>

Author: nerdile

CHANGELOG.next.toml

@@ -22,3 +22,9 @@ message = "Add support for v1 `http_body::Body` to `aws_smithy_types::byte_strea
 references = ["smithy-rs#1925", "smithy-rs#3673"]
 meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "all"}
 author = "Velfi"
+
+[[smithy-rs]]
+message = "Enable aws-smithy-runtime to compile in rustc 1.72.1"
+references = ["smithy-rs#3675"]
+meta = { "breaking" = false, "tada" = false, "bug" = true, "target" = "client" }
+author = "dastrom"

rust-runtime/aws-smithy-runtime/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "aws-smithy-runtime"
-version = "1.5.5"
+version = "1.5.6"
 authors = ["AWS Rust SDK Team <aws-sdk-rust@amazon.com>", "Zelda Hessler <zhessler@amazon.com>"]
 description = "The new smithy runtime crate"
 edition = "2021"

rust-runtime/aws-smithy-runtime/src/client/http/body/minimum_throughput.rs

@@ -219,7 +219,7 @@ impl UploadReport for ThroughputReport {
 pin_project_lite::pin_project! {
     /// Future that pairs with [`UploadThroughput`] to add a minimum throughput
     /// requirement to a request upload stream.
-    struct UploadThroughputCheckFuture {
+    pub(crate) struct UploadThroughputCheckFuture {
         #[pin]
         response: HttpConnectorFuture,
         #[pin]