GenericRecord: Test DeSes with nested structures in fields

[jens-guenther]

Test nested structures in GenericRecord fields like

• complex object
• array, set, list, map (others?) of simple objects (like String, Number, ...)
• collection (each of ...) of complex objects
• collection (each of ...) of collections

with all DeSes (json, avro, protobuf)

[yianni]

Please be advised that we have collectively agreed not to support Avro or Protobuf representation of GenericRecord.

[jens-guenther]

Ya, you're right. Let's check with Json, at least.

We need to document the fact that GenericRecord is not serializable using acro and protobuf.