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.*}