Add duo_workplan_async_flow feature flag definition
What does this MR do and why?
Adds the duo_workplan_async_flow feature flag definition on its
own, so it can land in master ahead of the full feature's review.
The flag will be used to enable an asynchronous, server-side flow for generating a work item Workplan through the Duo Agent Platform, as an alternative to the current interactive Duo Chat flow.
See #605157 for the feature this flag will gate.
Edited by Stefanos Xanthopoulos