[ATR-21817] Add trigger menu information to python meta reader

As discussed on [ATR-21817], the DB folder dumps of trigger metadata were removed in master in [ATR-20272]. These were a relic of Run1, and have been redundant in R2 as we write the trigger menu in xAOD format.

But these folders were still being used e.g. by the derivation code to configure based on primary triggers.

This MR populates the current metadata dictionary with a list of L1 items and another list of HLT chains. These come from the first SMK stored in the AOD.

Both are stored under the TriggerMenu key with L1Items and HLTChains keys, respectively.

The modifications to read the data are maybe a little ugly as this class wasn't really designed to be passing around both interface and auxiliary class data. cc @mnowak, @dciubota in case they want to comment.

In the near future, this will need more additions to populate from the new R3 xAOD JSON trigger metadata stores. But here we can keep the same format in the metadata dictionary.