cluster-configuration: Add kafka-operator configuration

[review commit by commit]

This adds the minimum required to run kafka within our kube clusters.

The first goal is currently to be able to start an environment from scratch with kafka ready.

To that end, this adapts the minikube cluster::

• *storage rpc backends writes to kafka (storage, indexer-storage, ...)
• journal-clients (scheduler, webhook, backfiller, ...) reads from the kafka backend to do their work

TODO:

• Actually activates through cluster-component the kafka cluster configuration (done manually for now, i need to develop out of it)
• Adapt the storage to write to the journal

Refs. swh/infra/sysadm-environment#5344 (closed)