The AvroCoder changes for
BEAM-2303 changes the reader/writer from the Avro ReflectDatum* classes to the SpecificDatum* classes.
Because of the way Avro handles Strings, however, the underlying instances for String data are deserialised as org.apache.avro.util.Utf8 instances instead of java.lang.String.
1. an unexpected behaviour change when migrating to Beam 2.30.0
2. potential serialization issues when using these String instances (Utf8 instances don't implement Serializable)
3. an inconsistent API between AvroCoder and AvroSink/AvroSource (the latter still use ReflectDatum*)