Statistics is not available for DECIMAL types

According to parquet format specification columns annotated as DECIMAL should use SIGNED comparator and statistics should be available. The sort order returned by `org.apache.parquet.format.converter.ParquetMetadataConverter` for DECIMAL is `SortOrder.UNKNOWN` which contradicts the specification and makes statistics for DECIMAL types unavailable.

**Reporter**: [Vlad Rozov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=vrozov) / @vrozov
**Assignee**: [Vlad Rozov](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=vrozov) / @vrozov
#### Related issues:
- [Allow for Unsigned Statistics in Binary Type](https://github.com/apache/parquet-format/issues/314) (is caused by)
#### PRs and other links:
- [GitHub Pull Request #494](https://github.com/apache/parquet-mr/pull/494)
- [Decimal format specification](https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#decimal)

_{**Note**: *This issue was originally created as [PARQUET-1322](https://issues.apache.org/jira/browse/PARQUET-1322). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

org.apache.parquet.format.converter.ParquetMetadataConverter から始め、DECIMAL のソート順がどのように決定されるかを調べてください。その動作を Decimal format specification および関連する統計情報の議論と比較してください。DECIMAL 列が指定された SIGNED コンパレーターを使用し、その統計情報を利用できるようになれば完了です。

索引モデルが issue の本文から書いたものです。