Send custom key-value pairs in ot-span-context to Zipkin

Thanks for maintaining the great project 👍

Envoy does seem to propagate ot-span-context but drops most of the key-values included in it before sending it to Zipkin.

Would it be possible to send those key-values as binary annotations of Zipkin spans?
Could I contribute it?

Context:

- [After looking into code](https://github.com/envoyproxy/envoy/blob/f48e0f91524776f4984d896f5b9cfe146591f1a1/source/common/tracing/zipkin/tracer.cc#L53-L114), it turns out that Envoy as of today drops all the keys in ot-span-context other than a few, well-known keys like traceid and spanid, before sending these to Zipkin.
- AFAIK, ot-span-context could be populated with "custom" keys and values other than these, e.g. by our apps, to provide additional context throughout multiple spans in single trace.
- If Envoy could translate those to corresponding binary annotations in Zipkin spans, we could view those custom key-value pairs from ot-span-context in Zipkin.
- FYI, an app like [dd-zipkin-proxy](https://github.com/flachnetz/dd-zipkin-proxy) allows Datadog to consume Zipkin spans containing arbitrary binary annotations so that it would also help Datadog users.