Add Trace Propagation to Kafka Events

[pbrisbin]

This library supports "automatic" trace-propagation via inject/extract/process functions that operate on any HasHeaders a, for which we have instances on Request (for HTTP) and Job (for Faktory).

We should do the same in our Kafka libraries.

My current design would be to make an Envelope around Value (or any a with JSON instances), and put the headers in that envelope. Then you can wrap on produce and unwrap on consume. The only nuance would be to make sure in-flight events that lack an envelope function fine (just without trace context).