Zod: add `default` when OpenAPI schema specifies a default value.

Given a schema with a `default` specified, I'd expect the generated Zod schema to have a fallback with the property `.default({value})`. That's not the case at the moment.

The current solution I've found is to have a post generation script do the heavy lifting, but that would be indeed a nice to have. Maybe behind a flag, to not strictly do a breaking change.