Implement AssignWindows for known WindowFns in the ReferenceRunner #18814
Description
This decodes known WindowFns from the portable representation if there is no associated environment, and runs those WindowFns on the input stream. As none of those known fns interact with the element directly, this is safe to do outside of an SDK harness.
Imported from Jira BEAM-4241. Original Jira may contain additional context.
Reported by: tgroh.