MDEV-23645: Optimizer trace: print conditions after substitute_for_best_equal_field

Print the conditions for WHERE, HAVING, and ON.

sql/opt_trace.h

@@ -108,6 +108,10 @@ void print_final_join_order(JOIN *join);
 void print_best_access_for_table(THD *thd, POSITION *pos,
                                  enum join_type type);
 
+void trace_condition(THD * thd, const char *name, const char *transform_type,
+                    Item *item, const char *table_name= nullptr);
+
+
 /*
   Security related (need to add a proper comment here)
 */