PostgreSQL: Support `PRIMARY KEY USING INDEX` syntax in `ALTER TABLE`

The parser fails to parse the PostgreSQL-specific `PRIMARY KEY USING INDEX index_name` syntax in `ALTER TABLE` statements:

```sql
ALTER TABLE version_downloads
ADD CONSTRAINT version_downloads_pkey PRIMARY KEY USING INDEX version_downloads_unique;
```

```
Expected: a list of columns in parentheses, found: INDEX
```

## Expected behavior

This is valid PostgreSQL syntax per the [ALTER TABLE documentation](https://www.postgresql.org/docs/current/sql-altertable.html). The grammar allows:

```
ADD table_constraint_using_index
```

Where `table_constraint_using_index` can be:

```
[ CONSTRAINT constraint_name ]
{ UNIQUE | PRIMARY KEY } USING INDEX index_name
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
```

Note that `PRIMARY KEY USING INDEX index_name` is a distinct form that does **not** require a column list - it promotes an existing unique index to be the primary key constraint.

Commencez par localiser la grammaire de ALTER TABLE et la gestion des contraintes de table dans le parseur SQL Rust. Utilisez l’exemple PostgreSQL de cette issue comme entrée de parsing, puis ajoutez une couverture pour PRIMARY KEY USING INDEX et confirmez que l’instruction est parsée sans exiger de liste de colonnes.

Rédigé par le modèle d'indexation à partir du texte de l'issue.