function_name was required to set, but it wasn't actually passed to module "lambda" inputs.
Allow callers to stop providing function_name and preserve old behavior of using automatically generated name.
When using Atmos to generate inputs from "stack" YAML files, having the ability to pass the
statements in as a custom object means specifying them via YAML, which makes the policy declaration in stack more
readable compared to embedding a JSON string in the YAML.