How to generate AWS RDS auth token

### Describe the issue

Previously there was some documentation at https://github.com/awslabs/aws-sdk-rust/blob/060d3c5a22a0b559cc459cbdbbe80b28685630c5/sdk/aws-sig-auth/src/lib.rs that defined how to generate some RDS token credentials.

Now that the code-base has been refactored I'm not sure how to do it?

There should be a helper function somewhere like in other AWS SDK's that provide easy access to this.

### Links

https://github.com/awslabs/aws-sdk-rust/blob/060d3c5a22a0b559cc459cbdbbe80b28685630c5/sdk/aws-sig-auth/src/lib.rs
https://docs.aws.amazon.com/cli/latest/reference/rds/generate-db-auth-token.html

(no longer relevant).

Start with the previously referenced sdk/aws-sig-auth/src/lib.rs and compare it with the current refactored authentication code. Read the AWS CLI generate-db-auth-token documentation to establish the expected token behavior, then determine where a Rust helper would belong and what interface should expose it. Done means the current SDK provides an easy way to generate AWS RDS auth tokens.

Written by the indexing model from the issue text.