AFT-411: Add enums for btag EDM schema

I don't love doing this but we have a bit of a conundrum in the b-tagging group. To avoid defining variables in a few places we've been adding dynamic variables to the EDM.

Unfortunately the names are rather ad-hoc and sometimes misleading, but fortunately they haven't propagated too far: at the moment they only live in a few flavor tagging derivations. The solution here is to phase them out by making the naming scheme configurable via an enum.

At some point we'll remove all the old names completely, but I'd like to give a grace period since we're currently in the middle of a retraining campaign and changing the names of all the variables would be quite disruptive.