Support dynamic timestamp args in Iceberg CALL

### Feature Request / Improvement

To support:
```sql
CALL DSv2HmsCatalog.system.expire_snapshots(
table => 'db.table',
older_than => CURRENT_TIMESTAMP() - INTERVAL 2 DAYS,
retain_last => 1,
except_replace_snapshot => false
);
```

### Query engine

Spark

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start by running the provided Spark SQL CALL example and trace how Iceberg handles its arguments, especially the older_than expression. Done means the example accepts CURRENT_TIMESTAMP() - INTERVAL 2 DAYS and applies the resulting timestamp when expiring snapshots.

Written by the indexing model from the issue text.