Add minimal flavor tagging links to JETM2

Someone tried to add flavor tagging to a JETM2 derivation and reported that it failed due to a missing jetLink from the b-tagging object to the jet. This is marked as a draft because obviously we need to discuss with @atlas-jsv, but it might be useful to be able to add the latest flavor tagging discriminants to the jet derivations.

This is more a starting point for discussion than a concrete proposal: there might be a few other things required.