Add support for Parameter and Secrets Lambda Extension cache

[selfsimilar]

https://aws.amazon.com/blogs/compute/using-the-aws-parameter-and-secrets-lambda-extension-to-cache-parameters-and-secrets/

AFAICT, this will require adding a new layer to the serverless.yml, which would be a documentation issue, and then code changes to support this. Currently this project already caches values, so it would be up to the maintainer to decide if the extension layer would replace the current cache function (and simplify the code), or act as an alternative. Happy to give some time to implementing this if you think this is a good idea.