Add Combat Timer as a DataBar widget

[KechhaTV]

What does this PR do?

Adds a new Combat Timer widget to the DataBars module.

The timer uses Blizzard's Damage Meter session data to display the current combat duration independently of the EllesmereUI Damage Meters module. It uses a combat-only heartbeat to keep the displayed duration updated, following the same approach as the existing Standalone Combat Timer in the Damage Meter module.

This brings the existing Combat Timer functionality to DataBars while allowing it to benefit from the DataBar system's existing layout, styling, and customization options.

How was it tested?

• Added the Combat Timer through the DataBars configuration.
• Verified that the timer starts updating when entering combat.
• Verified that the displayed duration matches the Blizzard Damage Meter session duration.
• Verified that the timer continues to work with EllesmereUI Damage Meters disabled.
• Verified that the timer stops updating when leaving combat and retains the final session duration.
• Verified that the DataBar can be removed/reconfigured normally.

Screenshots

Checklist

• New settings default OFF (no behavior change without opt-in)
• [N/A] Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built
  This does not fully apply to the DataBar implementation. Once the Combat Timer widget is added to a DataBar, its event frame is created and the relevant Damage Meter events are registered. The refresh heartbeat itself is only active while in combat and is stopped immediately when leaving combat. When the widget is not enabled/used, it has no associated events, polling, or update work.
• [N/A] Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations).
  This does not apply to this implementation. The Combat Timer intentionally follows the same update approach as the existing Standalone Combat Timer in the Damage Meter module. The purpose of this PR is to replicate that functionality as a DataBar widget, while making it available within the DataBar system for increased customization and integration with existing DataBar styling and layout options.
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames
• Tested in-game on live; no version gates or pre-Midnight APIs added