added RDS IAM Auth example

[Kevinwochan]

✍️ Description of changes: Added an example of how to use CDK to deploy a rust lambda function that uses IAM Authentication to connect to an RDS instance, feel free to close if out of scope of this repo.

🔏 By submitting this pull request

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

[Kevinwochan]

When the testing pipeline becomes more flexible, it'd be great to specify a config in the diretory to tell the testing script where the rust project is.

That way we can show what a good directory structure looks like when working with CDK

[calavera]

We're definitely open to improvements on those testing scripts if you want to take a pass in future PRs 🙌