Add event for AWS/SFTP Transfer - custom identity provider

I do not see anything related to AWS Transfer Family. In particular, they support using Lambdas as custom "authorizers" (identity providers), as you can see here: https://docs.aws.amazon.com/transfer/latest/userguide/custom-identity-provider-users.html

Currently, I need to implement my own request/response structs. It would be nice if they would be provided by this package.