Avoid exposing `Arc`ed `ResolveEndpoint` in public API (#2758)

## Motivation and Context
Hides `Arc<dyn ResolveEndpoint>` from public API.

## Description
This PR replaces the occurrences of `Arc<dyn ResolveEndpoint>` with
`SharedEndpointResolver` to not expose bare `Arc`s in the public API.

## 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

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