Can't use `sendAwait` with Kafka's MockProducer

[felixkrull-neuland]

sendAwait is defined directly on KafkaProducer. This makes it impossible to use with Kafka's own MockProducer. The method should be defined on the Producer interface instead.

[nomisRev]

Hey @felixkrull-neuland,

Thank you for opening this ticket, an alternative implementation has been written for producing elements that will be released soon. It is defined on top of Producer so it should fix this issue, but I will keep this open until I've released that in a next version.

You can check out the API, and implementation here.
Any feedback would be greatly appreciated, #156