Optimize `AggregatedBytes::to_vec` (#2786)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Avoid intermediate vec allocations in `AggregatedBytes::to_vec`.

## Description
<!--- Describe your changes in detail -->
Calling `slice::to_vec` on every segment is wasteful, the segments can
be just flattened into an iterator over `u8`s and collected directly.
Also makes the code a bit simpler.

## Testing
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to -->
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## Checklist
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rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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Co-authored-by: 82marbag <69267416+82marbag@users.noreply.github.com>