Clone TimeboxReportService

What does this MR do and why?

The feature update proposed in #381879 requires making a significant change to TimeboxReportService class. However, the service class is complex and cannot easily be updated in place.

In this MR,

• I am going to create a copy of TimeboxReportService so that I can modify it without touching the original in later MRs.

  • I am splitting the copy of TimeboxReportService into two new service classes Timebox::EventAggregationService and Timebox::RollupReportService.

    • In other words TimeboxReportService = Timebox::EventAggregationService + Timebox::RollupReportService (it's barely a refactor because I am merely shifting around the methods/attributes etc as they are.)

• Introduce a new FF rollup_timebox_chart.

  • When this FF is enabled, the consumer of TimeboxReportService will instead use Timebox::RollupReportService.

The purpose of the above changes is 1) to ensure we can continue to serve the original TimeboxReportService to users should we find regressions in the updated implementation and 2) to allow future reviewers to easily understand what new changes are being made to Timebox::EventAggregationService and Timebox::RollupReportService (it's better than cloning and making changes at the same time.)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.