Add SharedAsyncSleep wrapping Arc<dyn AsyncSleep> (#2742)

## Motivation and Context
Add `SharedAsyncSleep` wrapping `Arc<dyn AsyncSleep>` and update call
sites of `Arc<dyn AsyncSleep>` to use `SharedAsyncSleep`

## Description
Public APIs that take/return a trait object for `AsyncSleep` have been
using `Arc<dyn AsyncSleep>` in the codebase. This has a downside of
exposing a bare `Arc`, making it difficult to change the APIs in the
future without breaking backwards compatibility. This PR adds a newtype
wrapper `SharedAsyncSleep` that hides `Arc<dyn AsyncSleep>` to alleviate
the said issue (the idiom of having `SharedXXX` has been seen in the
codebase, e.g. `SharedTimeSource`, `SharedCredentialsProvider`, and so
on).

## Testing
- [x] Passed tests in CI

## 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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choice._

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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: John DiSanti <jdisanti@amazon.com>