Expose hyper 1.0 client builder behind not stable feature flag

### Describe the feature

I'm trying to upgrade from the hyper 0.14 impl to hyper 1.x impl for the HTTP client. I use to customize things like HTTP client buffer sizes to constrain or expand memory usage in certain cases (favoring memory usage vs transfer speed depending on the case). I would like to control similar settings on the hyper 1.x implementation.

I see per https://github.com/awslabs/aws-sdk-rust/discussions/1261 that you don't want to expose the hyper_util stuff because it's unstable. Fair enough. However, can you possibly expose the ability to pass in a hyper builder behind some kind of unstable feature flag on the crate? Then users can opt-in and suffer the consequences of future breakages. This seems like a very small change with low risk to you which otherwise forces everyone to make a fully custom HTTP client which seems like overkill.

Thanks for considering.

### Use Case

Customize the HTTP client without having to fully fork it.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### A note for the community

### Community Note
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Start by tracing where the Rust SDK constructs its hyper 1.x HTTP client and how crate feature flags are defined. Determine the supported boundary for exposing a hyper builder without making unstable hyper_util APIs part of the stable interface. Done means users can customize the builder behind an explicitly unstable feature flag, with relevant tests passing.

Written by the indexing model from the issue text.