Enhance current defacto standard for event enhancement of dead letter sink routing

[matzew]

Based on #102

There is interest in doing more, than those three attributes. See here knative-extensions/eventing-kafka-broker#2376

One way to address this, is to allow some standardized http header prefix, and pass all of those prefixed headers/values through, for more advanced CE decoration in the case that an event could not be delivered and routed to the DLS.