Explore adding rule to mark contrib as having been converted

My proposal is that rector adds this line when doing the hook conversion if it's not already there in YOURMODULE.services.yml

parameters:
  # Verify that all hooks have been converted, then set to true 
  YOURMODULE.hooks_converted: false

Or maybe even default to TRUE and change the comment slightly but that's maybe too risky.

IMHO, this is the only way how we'll get a sensible adoption of this flag in contrib and custom code. Nobody will otherwise find this and actively look it up and remember to add it.