Support overriding hyper executor to support runtimes other than tokio

### Describe the feature

There is no way to override the hyper executor in `aws-smithy-http-client` since the `hyper-util` client builder is not exposed (by design).

### Use Case

https://github.com/awslabs/aws-sdk-rust/discussions/1257#discussioncomment-13274634

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### A note for the community

### Community Note
* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue, please leave a comment

Start by reading the aws-smithy-http-client implementation that uses the hyper-util client builder, then review the linked discussion for the runtime requirements. Define how callers can override the hyper executor for runtimes other than Tokio and verify that the resulting client remains usable with the supported runtime choices.

Written by the indexing model from the issue text.