Hide scheduler API

The scheduling of jobs from different aspects can likely be hard wired for now.
Later when we add more aspects and users want to integrate their own we may need to somehow expose the scheduler. This could either be done imperatively and force the user to override a function and install a new scheduler or in a data driven way similar to how the framegraph works.

The latter lends itself more to a declarative API.

For the initial release, ensure that the scheduler API is private and that no API leaks out - e.g. the aspect type enum.