appendRowGroup will loose pageIndex

Currently, 
org.apache.parquet.hadoop.ParquetFileWriter#appendFile(org.apache.parquet.io.InputFile) uses appendRowGroup method to concate parquet row group. However, appendRowGroup method **looses** column index.
```java

// code placeholder
  public void appendRowGroup(SeekableInputStream from, BlockMetaData rowGroup,
                             boolean dropColumns) throws IOException {
....
    // TODO: column/offset indexes are not copied
      // (it would require seeking to the end of the file for each row groups)
      currentColumnIndexes.add(null);
      currentOffsetIndexes.add(null);
}
```
 

Look forward to functionality that support append with page index.

**Reporter**: [GANHONGNAN](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=NathanKan)

_{**Note**: *This issue was originally created as [PARQUET-2340](https://issues.apache.org/jira/browse/PARQUET-2340). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

从 parquet-hadoop/src/main/java/org/apache/parquet/hadoop/ParquetFileWriter.java 中的 appendFile(InputFile) 和 appendRowGroup 开始。跟踪行组列索引和偏移索引的表示方式，以及当前方法添加 null 条目的原因。检查周围的 writer 逻辑和现有测试，然后将完成标准定义为：追加的行组保留其页面索引。

由索引模型根据 Issue 内容生成。