[PWGEM] add switchable crossed-pair (leg-swap) veto with QA

[Steffimro]

The idea behind this study is to see whether I could reduce the amount of fake photons. This is done by using the photon pairs which have previously passed the delta eta cut and are re-paired and checked whether they could form a "fake photon".

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 69 warnings, 🔕 7 disabled

[alibuild]

Error while checking build/O2Physics/code-check for af193f4 at 2026-07-27 14:32:

## 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/82da527a447c8c1ba3f265ad9d6750f6ebd51401/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Tasks/photonhbt.cxx:742:32: error: no header providing "TH1" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/photonhbt.cxx:2138:18: error: no header providing "o2::soa::is_table" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 2 -gt 0 ]]
++ exit 1
--

Full log here.