[PWGJE] Reduce memory usage with conditional histogram initialization in jetDsSpecSubs

[monamelop]

• initialize histograms conditionally according to the enabled process;
• avoid allocating unused histograms;
• add support for an on-the-fly MC analysis process;
• preserve the existing analysis outputs;

[github-actions]

O2 linter results: ❌ 0 errors, ⚠️ 8 warnings, 🔕 0 disabled

[alibuild]

Error while checking build/O2Physics/code-check for 605e064 at 2026-08-05 11:45:

## sw/BUILD/O2-latest/log
CMake Error in Detectors/Base/CMakeLists.txt:

Full log here.

[alibuild]

Error while checking build/O2Physics/staging for 605e064 at 2026-08-05 11:53:

## sw/BUILD/O2-latest/log
CMake Error in Detectors/Base/CMakeLists.txt:

Full log here.

[alibuild]

Error while checking build/O2Physics/o2 for 605e064 at 2026-08-05 11:53:

## sw/BUILD/O2-latest/log
CMake Error in Detectors/Base/CMakeLists.txt:

Full log here.