hi there! I'm trying to fill some gaps in the java version of flytekit. I'd like to update the local-executor so more test can be written. I've opened this PR to address sub-workflows. It would be nice if someone can have a look and maybe even try it. If this is going the right way, I think mocking sub-workflow in tests shouldn't be too hard

cc @thankful-minister-83577 / @icy-agent-73298

Nice job @gorgeous-glass-72642 - Thanks !

@better-air-29727 @boundless-pizza-95864 ^

Here is the proposal to mock sub-workflows