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.*}

Beginne in parquet-avro/src/main/java/org/apache/parquet/avro/AvroIndexedRecordConverter.java beim FieldEnumConverter-Code um die Zeilen 236-238 und vergleiche dann das referenzierte Verhalten von GenericData.EnumSymbol in Avro's GenericData.java. Überprüfe, dass beim Lesen generischer Datensätze die Enum-Schemadarstellung erhalten bleibt, anstatt auf einen String zurückzufallen; die Aufgabe ist erledigt, wenn die resultierenden Datensätze dem Schema entsprechen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.