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

• 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.

[calavera]

This is great. Thanks a lot for providing such a complete example. My only concern is that with the current directory structure, the rust code base won't be tested with changes in the runtime, like all the other examples are.

Do you mind using a similar structure to the one in this example? https://github.com/awslabs/aws-lambda-rust-runtime/tree/main/examples/http-axum

We have a script that builds all the examples in every PR to verify that the code works, but it's being skipped in this one because of the directory structure: https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/examples/check-examples.sh

[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 🙌

[calavera]

Thanks!