[clients/avro] Avro ReflectData example

Sometimes Java classes can be complex. For example, you want a Date/Instant object, but then Avro doesn't have a date type, so it should actually be encoded as a long. Rather than having Producer & Consumer application logic to handle that conversion, it can be done by Encoders in the Reflection API

Provide example for [ReflectData](https://avro.apache.org/docs/1.8.2/api/java/index.html?org/apache/avro/reflect/ReflectData.html), which are subclasses of SpecificRecords, and are easier to migrate existing Jackson/Gson POJOs into this model rather than write up an Avro schema