@Override
  public String[][] listNamespaces() {
    return unifiedCatalog.listDatabases().stream()
        .map(d -> new String[] {d})
        .toArray(String[][]::new);
  }
Unified catalog implement for spark engine.
amoro-format-mixed/amoro-mixed-spark/amoro-mixed-spark-3-common/src/main/java/org/apache/amoro/spark/SparkUnifiedCatalogBase.java