FakeBkgTools: also allow ConstDataVector input container
So far one could only pass leptons to the tools via a const xAOD::IParticleContainer but it's been pointed out in !72538 (merged) that a ConstDataVector could be better in some scenarios (i.e. when lepton objects are retrieved from const containers). So this MR adds the necessary overload. This shares the same implementation as for IParticleContainer because only const methods were accessed.