public static StringBuilder number(StringBuilder buf, int v, int n) {
    for (int k = digitCount(v); k < n; k++) {
      buf.append('0');
    }
    return buf.append(v);
  }
Helper for CAST({date} AS VARCHAR(n)).
amoro-format-mixed/amoro-mixed-flink/amoro-mixed-flink-common/src/main/java/org/apache/amoro/flink/util/DateTimeUtils.java