expose sql.DB to let orm operate on it (#1015)

* expose sql.DB to let orm operate on it

* add missing RawDB methods

* add NewSqlConnFromDB for cooperate with dtm

tools/goctl/model/sql/test/sqlconn.go

@@ -13,6 +13,7 @@ type (
 	MockConn struct {
 		db *sql.DB
 	}
+
 	statement struct {
 		stmt *sql.Stmt
 	}
@@ -62,6 +63,11 @@ func (conn *MockConn) QueryRowsPartial(v interface{}, q string, args ...interfac
 	}, q, args...)
 }
 
+// RawDB returns the underlying sql.DB.
+func (conn *MockConn) RawDB() (*sql.DB, error) {
+	return conn.db, nil
+}
+
 // Transact is the implemention of sqlx.SqlConn, nothing to do
 func (conn *MockConn) Transact(func(session sqlx.Session) error) error {
 	return nil