Adds avro schema for Zipkin V2 model

[jcchavezs]

Currently we provide JSON and proto schemas for Zipkin V2 model but as Kafka ecosystem is becoming more integrated with Zipkin it would be good to have an avro schema.

Related to openzipkin-contrib/zipkin-storage-kafka#74 (comment)

cc @adriancole @jeqo @kotharironak

[jorgheymans]

Maybe the rationale could be elaborated upon ? Kafka as such does not care about avro.