[Common] Fill MC hists in a loop over MC collisions in CentralityQa

[romainschotter]

• Add option to loop over MC collisions to fill MC histograms, while before all MC histograms were filled in a loop over reconstructed collisions

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 25 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for d454833 at 2026-08-31 20:02:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 2 errors and 0 warnings.'
Found 2 errors and 0 warnings.
++ [[ 2 -gt 0 ]]
++ cat /sw/BUILD/ed84201d914a9c9e397dd0c5ce33eae0ca1fa71d/O2Physics-code-check/errors.txt
Common/Tasks/centralityQa.cxx:1437:13: error: variable 'nContribsInEta05' set but not used [clang-diagnostic-unused-but-set-variable,-warnings-as-errors]
Common/Tasks/centralityQa.cxx:1449:13: error: Value stored to 'nContribsInEta05' is never read [clang-analyzer-deadcode.DeadStores,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 2 -gt 0 ]]
++ exit 1
--

Full log here.