MINT-9526: Add mergeMapping function to Tools/Mapper

[rafaela-mansini]

This change adds a new helper function in the Tools/Mapper/Mapper class, which merges results from the map() and mapAll() functions into a given mapper.

Usage:

media: "obj['gallery'] ? mapper.mergeMapping('single_media_from_catalog', [
  ['map', obj['gallery']['packshot'], 'packshot'],
  ['map', obj['gallery']['eRetail']['main'], 'eRetail'],
  ['map', obj['gallery']['map'], 'map'],
  ['mapAll', obj['gallery']['eRetail']['nonBrand'], 'eRetailerNonBrand']
]) : null"