[PWGEM] PhotonMeson: Add task for event normalization on derived data

[jokonig]

• If only events with a certain number of photons (or any other kind of selection) are stored in the derived data, the table "PMEvSelBits" in the event selection task can store the event selection bits. Hence, the number of selected events before the specific selection on the number of photons etc. can be retrieved.
• This new task just loops over these event-selection bits and stores them in a histogram.
• For future: Also store it as a function of multiplicity etc.

[github-actions]

O2 linter results: ❌ 2 errors, ⚠️ 0 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 681f61e at 2026-09-15 09:58:

## sw/BUILD/O2Physics-code-check-latest/log
--
========== List of issues found ==========
++ echo 'Found 5 errors and 0 warnings.'
Found 5 errors and 0 warnings.
++ [[ 5 -gt 0 ]]
++ cat /sw/BUILD/cd18de5d4275378687aea11b601f93c2f64c4964/O2Physics-code-check/errors.txt
PWGEM/PhotonMeson/Tasks/eventNormalizationTask.cxx:25:1: error: included header TH1D.h is not used directly [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/eventNormalizationTask.cxx:38:45: error: no header providing "o2::framework::OutputObjHandlingPolicy" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/eventNormalizationTask.cxx:60:10: error: no header providing "size_t" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/eventNormalizationTask.cxx:61:21: error: no header providing "TH1" is directly included [misc-include-cleaner,-warnings-as-errors]
PWGEM/PhotonMeson/Tasks/eventNormalizationTask.cxx:75:3: error: no header providing "PROCESS_SWITCH" is directly included [misc-include-cleaner,-warnings-as-errors]
++ [[ 0 -gt 0 ]]
++ [[ 5 -gt 0 ]]
++ exit 1
--

Full log here.

[vkucera]

@jokonig Your PR introduces errors. Please fix them before merging.