Fix for L1 streamer generation
- added (temporary!) old-style trigger flag to choose between xml and json as primary L1 source
- Implement reading of the L1 menu xml file reading during L1-streamer generation in the HLT menu. ==> Hence the items that are seeding chains like "HLT_noalg_Standby" are now taken from the XML menu, which is the same as the CTPUnpacker uses.
- The commit that adds the streamers is picked from @astruebi
- There is also a change in the signature monitoring so it can deal with multi-item-seeded chains (for these chains the WARNING about non-existing seed items has been demoted to INFO due to the current inconsistencies between xml and json menu)
Related to ATR-21343