Improve the interface for sharing complex data between steps / setting workflow-scoped variables

I author workflows that use multiple steps, either because I'm using pre-existing actions, or because I separate code in steps to keep my code / step interfaces clean.

These step boundaries carry reusability and observability benefits, but I also often pass the same params to steps more than once. Therefore, I want to define a variable once and pass it to future steps by reference.

Today, I can create a single, `CONFIG` or `CONSTANTS` step at the top of my workflow that exports these variables, but those step exports must be JSON-serializable. Therefore, I can't pass functions or complex constructors (for example).

I'd like a way to set arbitrary, workflow-level variables that I can re-use in future steps.