add an option to register callback on quantifier instantiation

Suppose a user propagator encodes axioms using quantifiers and uses E-matching for instantiation. If it wants to implement a custom priority scheme or drop some instances based on internal checks it can register a callback with quantifier instantiation

src/solver/tactic2solver.cpp

@@ -115,6 +115,10 @@ public:
         m_tactic->user_propagate_register_diseq(diseq_eh);
     }
 
+    void user_propagate_register_on_binding(user_propagator::binding_eh_t& binding_eh) override {
+        m_tactic->user_propagate_register_on_binding(binding_eh);
+    }
+
     void user_propagate_register_expr(expr* e) override {
         m_tactic->user_propagate_register_expr(e);
     }