Support ALTER TABLE RENAME TO, ADD SYNONYM

`ALTER TABLE RENAME TO, ADD SYNONYM` is not yet supported.

https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#alter_table

```
ALTER TABLE table_name
    action

where action is:
...
    RENAME TO new_table_name [, ADD SYNONYM synonym]
...
```

Related: #115 