[Refactor] Audit and prune redundant planning enums

[milindsrivastava1997]

I audited possibly unnecessary enums. I am unclear on whether we need these enums right now

Enums

• BoundaryKind?
• CacheProfile?
• SummaryWindowFramework::ExponentialHistogram and Extension(String)
• EvaluationSchedule
• QueryTimeScope
• Predictability

Other simplifications to make

1. Remove ExecutableOperator and the duplicated ExecutableDagNode.operator; derive the operator kind from ExecutableOperatorPayload
2. Remove the planner-local CostUnit in favor of the shared CostUnit enum.
3. Remove MaterializationMedium if no downstream consumer distinguishes memory, disk, and object-store materialization

Blocked by

None - can start immediately