Add AppConfig feature flags example

[calavera]

📬 Issue #, if available:

N/A

✍️ Description of changes:

This example shows how to integrate AppConfig with Rust Lambda functions. It includes CDK constructs to deploy the basic AppConfig scaffolding, and the AppConfig Lambda extension to reduce the latency fetching the AppConfig configuration.

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I confirm that I've ran cargo clippy --fix.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.