Adding keywords to evgen metadata

Keywords in event generation specify various things about the process being generated - what physics it might represent, what particles are being generated, and so on.

These keywords are extremely handy downstream for auto configuration. See for example the discussion in !67455 (merged). Similarly, one can configure SUSY helpers for all SUSY samples based on keyword, or Higgs helpers for all Higgs samples, and so on.

This adds the keywords to the EVNT metadata, so that they can be transferred along and accessed downstream. It'll be a long time before all our evgen have these keywords, but better to start the process now.