Stream implementation (wrapper) for PaginationStream (#3299)

## Motivation and Context
https://github.com/awslabs/aws-sdk-rust/discussions/995



## Description
I tried to implement futures::Stream for a wrapper struct around
`PaginationStream`. I am unsure if I did it in the best way. After
fighting with the borrow checker for a while I decided to try
`Arc<Mutex<_>>` - is this the way to go or does there exist a better
way?
Even then, does the code look correct? I used it in my project and my
integration tests do pass but I am not 100% that these tests will catch
any error in paginated ListObjectsV2.

I would appreciate any feedback so far.

## Testing
In progress while waiting for feedback on code

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates


_By submitting this pull request, I confirm that you can use, modify,
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choice._

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Co-authored-by: Russell Cohen <rcoh@amazon.com>
Co-authored-by: Russell Cohen <russell.r.cohen@gmail.com>