Consider providing .proto for all API specs

[dio]

Currently, zipkin APIs are represented as:

1. v1, http-json, through https://github.com/openzipkin/zipkin-api/blob/master/zipkin-api.yaml
2. v2, http-json, through https://github.com/openzipkin/zipkin-api/blob/master/zipkin2-api.yaml and also we have http-proto with messages defined in https://github.com/openzipkin/zipkin-api/blob/master/zipkin.proto

To make it seamless and take advantage of protobuf tooling when making zipkin clients, I think it is a good idea to have .proto for all available API specs (including v1 and v2 http json). E.g. in Envoy we can purely use protobuf helps without a detour on using json lib like rapidjson.

We can consider bringing: https://github.com/nytimes/openapi2proto to generate .proto from openapi spec.

[codefromthecrypt]

didn't notice this issue existed #77 partially implements it. v1 is not reverse mapped yet