private AppendFiles newAppendFiles(Transaction transaction) {
      if (fastAppend) {
        return transaction.newFastAppend();
      } else {
        return transaction.newAppend();
      }
    }
Implementation of {@link AppendFiles} for mixed-format table, adding tracing and watermark
generating logics.
amoro-format-iceberg/src/main/java/org/apache/amoro/op/MixedAppendFiles.java