Support ALTER STATISTICS statement

`ALTER STATISTICS` statement is not yet supported.

## Syntax

https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#alter-statistics

```
ALTER STATISTICS package_name
    action

where package_name is:
    {a—z}[{a—z|0—9|_|-}+]{a—z|0—9}

and action is:
    SET OPTIONS ( options_def )

and options_def is:
    { allow_gc = { true | false } }
```