[RFC?] context_creation mypy
Summary: Attempted to annotate context_creation_pipeline and associated items. This one gets a bit funky with generics and generators, so I would like some feedback on my approach. Overall though, I think the types help here a lot to illustrate what's going on with the EventGenerationManager stuff, which is a pretty arcane code path.
Test Plan: mypy
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D7571