Support Avro's unions with logical types

Avro 1.9.x introduced yet another mechanism for registering/looking up conversions for logical types. See https://issues.apache.org/jira/browse/AVRO-1891

If a logical type is part of a union a static field MODEL$ of type SpecificData will be added to the generated Avro class with registered conversions for a logical type. We should use that SpecificData in AvroSerializer and Avro(De)SerializationSchema whenever available.