Add go.mod in kafka directory

[acgtun]

Add modular in kafka directory, so that we could import only kafka modular without the other modulars.

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
✅ acgtun
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}

[milindl]

Hi @acgtun , I think this idea is worth looking into in more details and separate other modules (schemaregistry) but at the current time, I think this will break backwards compatibility by requiring every user to change their import statements (v2/kafka changes to kafka/v2). So we can't merge this immediately.
I'll discuss this internally to see if this is something we do with our next major version upgrade whenever it is planned.

[acgtun]

Thanks @milindl for looking into it.

[acgtun]

Hi @milindl, for us, the main reason we want to create a kafka modular is to reduce some of the dependency requirements.
for example, github.com/docker/docker , k8s.io/cri, etc, these that are not used in kafka modular.