Support distributed tracing in NATS handler

[Knappek]

Implement #281 and #311 for the NATS handler. This includes:

• Fontend -> RIG -> NATS (message): RIG processes trace context of the frontend from the header. As it is a message, RIG can't populate it to the message payload (could be a raw message, not even having cloudevent format). With Kafka, we will use Kafka headers (implemented in Support publishing W3C Trace Context in Kafka header #311), with NATS we have to think how to realize it
• Backend->NATS->RIG (events): RIG should read trace context from the cloudevent