address #223: prefer ADL over if constexpr for customization

Switch to specific ADL customization points that allow classes to 'opt-in' -- which is simpler and more scalable than using lots of if constexpr branches to adapt to the quirks of various classes.

requires LHCb!3338 (merged) (but not the other way around, eg. LHCb!3338 (merged) does not require this MR)