Add `aws-lambda` as a feature to generated server SDK (#3648)

## Motivation and Context
The generated SDK conditionally re-exports `routing::LambdaHandler`
under the feature flag aws-lambda, but the `Cargo.toml` does not define
such a feature.

## Description
Closes: https://github.com/smithy-lang/smithy-rs/issues/3643



## Checklist
- [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: Fahad Zubair <fahadzub@amazon.com>
Co-authored-by: david-perez <d@vidp.dev>