[clients/avro] Avro ReflectData example

[OneCricketeer]

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, which are subclasses of SpecificRecords, and are easier to migrate existing Jackson/Gson POJOs into this model rather than write up an Avro schema