feat: Added feature flag for kafkajs instrumentation

[bizob2828]

Description

Adds a feature flag for kafkajs so we can ship things dark while we're working through the instrumentation.

[elepedus]

Hey, thanks for working on this integration. Could you tell me what I'm supposed to see in NewRelic once the feature flag is enabled?

I was hoping I could remove the manual calls to startBackgroundTransaction in my message handlers, but that doesn't seem to be the case, and I'm not sure what metrics this is exposing, or what else I need to do to make it work.

[bizob2828]

@elepedus this example application demonstrates all the telemetry kafkajs provides