[Prism] Refactor stageState to a behavior interface to reduce branch combinatorics

[lostluck]

A basic no-behavior change refactoring to split the different stage kinds up more cleanly into the 3 types: ordinary, Stateful, and aggregation.

While their differences don't come up everywhere, they in particular affect when building bundles, and adding pending input elements to their stages.

• Ordinary stages are the simplest, and handle most simple batch stages.
• Stateful stages handle Stateful transforms, with state, timers, and a requirement for keys. They don't respect triggers.
• Aggregations are Stateful stages that don't handle timers, have specific conditions for triggering, as well as handling late data.

The hope of this change is to reduce the combinatorics complexity parts of prism were suffering under, depending on which kind of stage was being handled. This leads to some duplication between Stateful and aggregation stages, but having clear handling of their differences should make up for it.

This is a minimal change. Not every potential portion is moved over right now, just those that had additional branching paths due to the old Stateful or aggregate booleans. Future changes should move handling of the pending elements stacks into similar methods instead of simply checking both.

Part of prism work: #29650 and was originally mentioned in #33881

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[lostluck]

Alas, when I synced before starting this, my local builds stopped working, so I'm unable to dig into what I messed up in the migration (likely in the startEventTimeBundle handling). I'll be on a trip for a week, so I'll revisit after that.