Not able to do backwards compatible projection

Currently the following scenarios fail while using projections. This makes it hard to do backwards compatible schema changes like adding an optional field or making a required field optional.

1. Adding an optional field to an existing thrift schema and making it part of the projection for reading data written using older thrift.

2. Projecting a required field using optional.

**Reporter**: [Mouli Mukherjee](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=moulim)
#### PRs and other links:
- [GitHub Pull Request #558](https://github.com/apache/parquet-mr/pull/558)

_{**Note**: *This issue was originally created as [PARQUET-1464](https://issues.apache.org/jira/browse/PARQUET-1464). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

审查 issue 中描述的 projection 场景，尤其是添加可选的 Thrift 字段，以及将必需字段 projection 为可选字段。先阅读 GitHub Pull Request #558 和链接的迁移文档，然后使用这些材料确定受影响的 projection 行为，以及仍缺少哪些兼容性测试。

由索引模型根据 Issue 内容生成。