Adding auto-configuration for MC21a PRW

This MR adds some auto-configuration to take care of MC21a pileup reweighting following the discussion in ATLASG-1628.

The idea here is to hard-code files for the time being, since we have no Run 3 data that things could possibly correspond to. Once we have some data, these dummy files can be replaced by the real 2022 mu files, and the additional configuration flags can be gotten rid of.

This isn't the most beautiful thing I've ever done, and alternative solutions are welcome if someone spots something more elegant.