@Override
  public SourceReader<T, MixedFormatSplit> createReader(SourceReaderContext readerContext) {
    return new MixedFormatSourceReader<>(
        readerFunction, readerContext.getConfiguration(), readerContext, dimTable);
  }
generate mixed-format watermark. This is only for lookup join mixed-format table, and
mixed-format table is used as build table, i.e. right table.
amoro-format-mixed/amoro-mixed-flink/amoro-mixed-flink-common/src/main/java/org/apache/amoro/flink/read/MixedFormatSource.java