Enums are incorrectly handled by parquet-avro when using GenericRecords

It appears that enums are handled incorrectly when reading parquet as generic records.

Looking at the code:
https://github.com/apache/parquet-mr/blob/master/parquet-avro/src/main/java/org/apache/parquet/avro/AvroIndexedRecordConverter.java#L236-L238

FieldEnumConverter falls back to a string representation when it can't find the corresponding enum class. This is problematic when trying to read parquet files generically without specific records on the classpath because the records will no longer match the schema. I believe a more correct approach would be to wrap the enums in GenericData.EnumSymbol:
https://github.com/apache/avro/blob/master/lang/java/avro/src/main/java/org/apache/avro/generic/GenericData.java#L397

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

_{**Note**: *This issue was originally created as [PARQUET-557](https://issues.apache.org/jira/browse/PARQUET-557). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

从 parquet-avro/src/main/java/org/apache/parquet/avro/AvroIndexedRecordConverter.java 中第 236-238 行附近的 FieldEnumConverter 代码开始，然后对比 Avro 的 GenericData.java 中所引用的 GenericData.EnumSymbol 行为。验证 generic record 读取会保留 enum schema 表示，而不是回退为字符串；当生成的 records 与 schema 一致时，该 issue 即完成。

由索引模型根据 Issue 内容生成。