[Lambda] Rust SDK for Durable Functions

### Community Note

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**Tell us about your request**

Rust SDK support for [Durable Functions](https://aws.amazon.com/about-aws/whats-new/2025/12/lambda-durable-multi-step-applications-ai-workflows/) - even basic support for just `step` and `wait` would be great, and `waitForCallback` would be a stretch goal!

**Which service(s) is this request for?**

Lambda Durable Functions

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

We use Rust exclusively in our Lambda runtimes, and would like to take advantage of Durable Functions soon.

**Are you currently working around this issue?**

We're not, we're currently blocked from adopting Durable Functions in our Rust codebases.

issue 内の Durable Functions リンクから始め、step と wait に必要な Rust SDK のエントリポイントを定義します。waitForCallback は追加目標として扱います。合意したスコープが動作する形でサポートされれば完了ですが、このロードマップ issue ではファイル、テスト、実装エントリポイントは指定されていません。

索引モデルが issue の本文から書いたものです。