Support Query and DML in both of ExecuteStreamingSql and ExecuteSql

[apstndb]

Currently, handy-spanner supports only DML for ExecuteSql and only queries for ExecuteStreamingSql.
In the real instance, the both of ExecuteSql and ExecuteStreamingSql can execute the both of queries and DMLs.

It makes handy-spanner incompatible with the current behavior of spanner-cli.
(THEN RETURN support needs to execute DMLs using ExecuteStreamingSql in the current Cloud Spanner client library.)
cloudspannerecosystem/spanner-cli#151

spanner> INSERT INTO Counter(pk, col) VALUES(0, 0);
ERROR: spanner: code = "InvalidArgument", desc = "Syntax error: \"INSERT INTO Counter(pk, col) VALUES(0, 0)\": syntax error::1:1: expected token: SELECT, but: <ident>\n\n  1:  INSERT INTO Counter(pk, col) VALUES(0, 0)\n      ^~~~~~\n"