[Feature Request] Orchestration Auto-Purge

** Let me know if this is not the right place to request a feature and I'll be happy to move.

It would be really nice to have a configuration setting in `host.json` to have orchestrations automatically clean-up themselves similar to how the `PurgeInstanceHistoryAsync` method works today.

I'm sure there are many production applications in-flight as we speak that overlooked this and would rather have this happen in the background.

Thanks and keep up the great work on this awesome product!