Parquet protobuf: List wrapper should be optional

Following  , parquet-protobuf will conform to the specs[1] regarding the definition of lists and will adopt the 3-level structure:
```java

*required* group my_list (LIST) {
repeated group list {
optional binary element (UTF8);
}
}
```
The first level has the repetition level **required**, however this won't allow making the distinction between a null list and an empty one.

This ticket is for changing the root level repetition to **optional** as following:
```java

*optional* group my_list (LIST) {
repeated group list {
optional binary element (UTF8);
}
}
```
This will allow differentiating between null and empty lists.

[1] https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#lists

**Reporter**: [Benoit Hanotte](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=b.hanotte)

_{**Note**: *This issue was originally created as [PARQUET-1216](https://issues.apache.org/jira/browse/PARQUET-1216). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

首先定位 parquet-protobuf 对列表 schema 的实现，并将其与 issue 中链接的 Parquet lists specification 以及所引用的 parquet-mr 更改进行比较。当 root LIST group 为 optional，同时嵌套的 list 和 element 结构保持如图所示，从而能够区分 null 列表和空列表时，即表示完成。

由索引模型根据 Issue 内容生成。