Support pure consumer processors

[flazarus1A]

Until now, the extension has been generating topologies for processors consuming and producing messages.

In #155, @melloware was interested in it but for a consumer-only use case.

We could adapt the TopologyProducer to be more lenient on the output side of things to accommodate Processor<K, V, Void, Void> cases.

[melloware]

This would be great!

[flazarus1A]

After some quick checks, it appears we have no gap for this. No unexpected shenanigans related to the absence of output topics/serializers.
I will enrich the project with a code sample and some tests, but the current implementation already permits not to produce anything.