feat(databars): add bags block

[Caedis]

What does this PR do?

Shows free/used bag slots with optional reagent bag and a low space threshold that recolors the count.

How was it tested?

Forever 1.60.1 (70009)
Add Bags datablock, configure as needed, sell/buy item to test threshold.

Screenshots

Threshold set to 21

Checklist

• New settings default OFF (no behavior change without opt-in)
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations)
• 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