Adding a few jet and event variables to prepare for automatic JEC workflows

[rappoccio]

To allow the JERC groups to use the NANOAOD, in preparation for automatic JEC workflows, we are adding:

1. Two variables with precision 6 to the AK4 jet collection (charged hadron fractions from PV and non-PV, with a delta-R definition instead of clustering definition).
2. Four event-wide floats (pileup density, gen-level pileup density, and two additional rho variables).

This will require the integration of a C++ module into CMSSW, PRs are here:

• 9.4.x: Adding some beta-star variables for JEC workflows. Intended for NANOAOD. Backport of #25223 to 9.4.x cms-sw/cmssw#25225
• 10.2.x: Adding some beta-star variables for JEC workflows. Intended for NANOAOD. Backport of #25223 to 10.2.x cms-sw/cmssw#25226
• 10.3.x: Adding some beta-star variables for JEC workflows. Intended for NANOAOD. Backport of #25223 to 10.3.x cms-sw/cmssw#25224
• 10.4.x: Adding some beta-star variables for JEC workflows. Intended for NANOAOD cms-sw/cmssw#25223