Revamp perf benchmarks: insertmanyvalues scenario, baseline comparison, CI artifact pipeline

- Add insertmanyvalues scenario (100K rows, 1000/batch, GH #500 repro)
- Add --baseline flag for 3-way comparison (main vs PR vs pyodbc)
- Add --json flag to save results for pipeline artifact consumption
- Publish benchmark results as CI artifact on main merge
- Download baseline artifact on PR runs for automatic regression detection
- Fix timing: time.time() -> time.perf_counter()
- Move connection setup outside timing window (measure execute+fetch only)

Author: bewithgaurav