[PWGDQ] Refactor FIT detector filling and add background pileup info

This commit refactors the FIT detector filling implementation to:
1. Remove unused FillFIT function that took individual detector collections
2. Simplify FillFIT for ReducedFIT tables to only read pre-computed columns
3. Add new BC-based FillFIT that processes all FIT detectors (FT0, FV0A, FDD) in one call
4. Implement background pileup calculation by scanning ±16 BCs following PWGUD pattern
5. Calculate distance to closest BCs with specific triggers (TOR, TSC, TVX, V0A, T0A)
6. Remove unused type traits (is_FT0, is_FV0A, is_FDD)
7. Optimize tableMaker_withAssoc.cxx to call FillFIT once per event instead of three times

Key changes:
- VarManager::FillFIT now has two clean overloads: one for ReducedFIT tables, one for raw BC data
- Background/beam-beam pileup flags stored as bit-packed integers (kBGFT0Apf, kBBFT0Apf, etc.)
- Distance variables track nearest BC with specific triggers within scan range
- Reduced code complexity and improved performance

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: minjungkim12