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.*}

从 org.apache.parquet.format.converter.ParquetMetadataConverter 开始，检查 DECIMAL 的排序顺序是如何确定的。将该行为与 Decimal 格式规范以及相关的统计信息讨论进行比较；当 DECIMAL 列使用指定的 SIGNED 比较器且其统计信息可用时，即表示完成。

由索引模型根据 Issue 内容生成。