Support DMLs in ExecuteStreamingSql

[apstndb]

I am not familiar with this codebase so I don't know it is preferred fix, but I verified that it can fix spanner-cli incompatibility.

$ spanner-cli --project fake --instance fake --database fake                                                    
Connected.
spanner> CREATE TABLE Counter(pk INT64, col INT64) PRIMARY KEY(pk);
Query OK, 0 rows affected (0.00 sec)

spanner> INSERT INTO Counter(pk, col) VALUES(0, 0);
Query OK, 0 rows affected (0.00 sec)

spanner> SELECT * FROM Counter;
+----+-----+
| pk | col |
+----+-----+
| 0  | 0   |
+----+-----+
1 rows in set (0.183 msecs)

#92