Add generic handlers of distributed trace headers

[nickdichev-firework]

Adds handlers for "generic" (I see this term used elsewhere in this module) tracestate and traceparent headers for the non-HTTP path of associating a trace.

My use case is associating these headers when my app consumes them off a Kafka message. The current API seems flexible enough for me to basically tell the agent they came from HTTP -- but this PR makes things a little more generic.

Should address #265

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