feat: add the rule to use-das when has variable assignments in sql

Co-authored-by: leslieyuchen <leslieyuchen@gmail.com>

src/sql/session/ob_sql_session_info.h

@@ -1150,6 +1150,7 @@ public:
   bool is_in_range_optimization_enabled() const;
   int is_better_inlist_enabled(bool &enabled) const;
   bool is_index_skip_scan_enabled() const;
+  bool is_var_assign_use_das_enabled() const;
 
   ObSessionDDLInfo &get_ddl_info() { return ddl_info_; }
   void set_ddl_info(const ObSessionDDLInfo &ddl_info) { ddl_info_ = ddl_info; }