FTAG GNN inference cleanup

• Improve function and type naming (e.g. "getters" "custom getters" etc)
• Improve type aliasing
• Start work on the V2 onnx metadata version. More to follow, see https://its.cern.ch/jira/browse/AFT-730

PhysicsAnalysis/JetTagging/FlavorTagDiscriminants/FlavorTagDiscriminants/ConstituentsLoader.h

@@ -76,7 +76,7 @@ namespace FlavorTagDiscriminants {
               m_config = cfg;
             };
             virtual ~IConstituentsLoader() = default;
-            virtual std::tuple<std::string, input_pair, std::vector<const xAOD::IParticle*>> getData(
+            virtual std::tuple<std::string, Inputs, std::vector<const xAOD::IParticle*>> getData(
                 const xAOD::Jet& jet, 
                 [[maybe_unused]] const SG::AuxElement& btag) const = 0;
             virtual FTagDataDependencyNames getDependencies() const = 0;