FTAG: add another truth label for light jet studies & improve alg config

This MR makes three changes:

• updating the ftagTruthTypeLabel to include a category for lambdas
• adding a new ftagTruthSourceLabel (someone please let me know if you have a better name), which gives info about the immediate parent of the TP (rather than the ftagTruthOriginLabel which gives origin info further up the chain in general). This is added to facilitate studies of mis-tagging in light jets.
• adds configurable accessors which is needed for re-running the labelling downstream (see atlas-flavor-tagging-tools/training-dataset-dumper!556 (merged))

cc @fdibello