Refactor RollupService.handleRollups

[andyjb]

In order to prepare for providing an option to process rollups using CDC/Eventing, the logic around providing the data and mechanism for calculating the rollups needs to be adjusted.

CDC will provide different information rather than Trigger.oldMap and Trigger.newMap values. The handleRollups method needs to become a generic processor of known steps regardless of whether CDC is being used or not, which lends itself to a factory pattern.

The common operations across implementations need to be identified, and an abstract class created containing them. A concrete class for the current way needs to be created, extending the abstract class of course.

Decision logic should be added into RollupService to allow the selection of the correct implementation.

For now, there's no requirement to implement the CDC version, this will be created separately.

Ensure that all tests continue to pass as before.