Provide hlt menu configuration as conditions object

This provides the HLT menu configuration as conditions data (ATR-20088)

I have not implemented the reading-back on the HLT side, however, I've updated the HLTChain class to deal with the new json format for chains (l1thresholds and groups).

The menu file is taken from the usual TriggerFlags, so the menu has to be set before the L1Sequence is called (hence the modification to TrigUpgradeTest/share/full_menu.py) At the moment the menu provider ignores the case when the json file is missing as there are two many instances where this is the case.