Draft: Adding truth_barcode for signal

Saving the truth_barcode for all hits.

Things to discuss

• Currently saving first barcode - in all of the cases I saw there is only 1 barcode but there could be cases with more. Ideally we would save a list but I'm unable to make it compile with a list maybe @dguest knows how to do it
• Do we also want to add some logic to already save the pdgId or a label (like isFromB) for the hits which have a barcode @svanstro suggested that we do it later (but maybe having it here would be an opportunity to harmonise)
• CI fails because truth_barcode is not in the test file we have - we probably need to make a new one from the ttbar sample - however that will also fail before is !508 (merged) merged

tag @mtanasin

see also atlas/athena!69866 (merged)