Spanner Dialect - ALTER INDEX Unexpected Error

Hello,
According to documentation, the ALTER INDEX statement should follow this example:
```
ALTER INDEX index_name {ADD|DROP} STORED COLUMN column_name
```
Reference: https://cloud.google.com/spanner/docs/secondary-indexes#googlesql_5

That said, while trying out the parser in the following piece of code
```
String query = ;
LanguageOptions languageOptions = new LanguageOptions();
ASTNodes.ASTScript parsedScript = Parser.parseScript(query, languageOptions);
```

I found that:

- When `query` was:
```
ALTER INDEX i1 DROP STORED COLUMN author;
```

The Parser returns: `Unexpected keyword STORED [at 1:21]`

- When `query` was:
```
ALTER INDEX i1 DROP COLUMN author;
```

The Parser returns: `ALTER INDEX is not supported [at 1:7]`

- When `query` was:
```
ALTER INDEX i1 ADD COLUMN author;
```

The Parser returns: `Unexpected ";" [at 1:33]`

- When `query` was:
```
ALTER INDEX i1 ADD STORED COLUMN author;
```

The Parser returns: `Unexpected keyword STORED [at 1:20]`

I was wondering if this was the expected result and if `ALTER INDEX` will be supported in the future.
Thanks