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/tactic/dependent_expr_state_tactic.h

@@ -168,6 +168,7 @@ public:
         m_frozen.push_back(e);
     }
 
+
     void user_propagate_clear() override {
         if (m_simp) {
             pop(1);