ProtoMessageConverter fails for unknown proto fields

Hi

We have Spark application that reads parquet files and turns them into a Protobuf RDD like the code below [1]. However, if the parquet schema contain fields that doesn't exist in protobuf class an IncompatibleSchemaModificationException [2] is thrown.

For compatibility reasons it would be nice to make it possible to ignore fields instead of throwing an exception. Maybe as an configuration? The fix for ignoring fields is quite easy, just instantiate an empty PrimitiveConverter instead.

Cheers,
-Kristoffer

[1]
JobConf conf = new JobConf(ctx.hadoopConfiguration());
FileInputFormat.setInputPaths(conf, rawPath);
ProtoReadSupport.setProtobufClass(conf, Msg.class.getName());
NewHadoopRDD rdd =
new NewHadoopRDD(ctx.sc(), ProtoParquetInputFormat.class, void.class, Msg.class, conf);
rdd.toJavaRDD().foreach(log -> {
System.out.println(log._2);
});

[2] https://github.com/apache/parquet-mr/blob/master/parquet-protobuf/src/main/java/org/apache/parquet/proto/ProtoMessageConverter.java#L84

[3] converters[parquetFieldIndex - 1] = new PrimitiveConverter() {};

**Reporter**: [Kristoffer Sjögren](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=deephacks) / @krisskross

_{**Note**: *This issue was originally created as [PARQUET-697](https://issues.apache.org/jira/browse/PARQUET-697). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Comienza en parquet-protobuf/src/main/java/org/apache/parquet/proto/ProtoMessageConverter.java, en el manejo señalado alrededor de la línea 84, y reproduce la IncompatibleSchemaModificationException con un campo de Parquet ausente en la clase protobuf. El trabajo estará terminado cuando los campos desconocidos puedan ignorarse sin la excepción, mientras el comportamiento de conversión existente permanece sin cambios; el issue no indica ningún archivo de prueba.

Escrito por el modelo de indexación a partir del texto del issue.