Remove third party types from public APIs Part 2 (#2848)

## Motivation and Context
Addresses item 8 in #2413 

## Description
This PR removes the third party types as follows:
- removes `SegmentedBuf` from public API in `aws-smithy-http` (used when
the feature `event-stream` was enabled)

## 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: ysaito1001 <awsaito@amazon.com>