Allow defining topic prefix for Kafka Broker and KafkaChannel

[pierDipi]

Problem

Currently, topic names are auto-generated with a prefix based on the resource name and namespace.

• https://github.com/knative-sandbox/eventing-kafka-broker/blob/898e479be20128819351ca341aff9d4e6f8ba50e/control-plane/pkg/reconciler/broker/broker.go#L52-L54
• https://github.com/knative-sandbox/eventing-kafka-broker/blob/898e479be20128819351ca341aff9d4e6f8ba50e/control-plane/pkg/reconciler/channel/channel.go#L63-L64

This makes difficult to use the same Apache Kafka cluster with multiple Knative Eventing installations in different Kubernetes clusters.

Similarly to 293b798, we could define in config-kafka-features topic names template:

brokers.topic.template: "knative-broker-{{ .Namespace }}-{{ .Name }}"
channels.topic.template: "knative-messaging-kafka.{{ .Namespace }}.{{ .Name }}"

and then use those templates for the underlying topic of Kafka Broker or KafkaChannel.

This will allow users to change the template for each Eventing installation.

Persona:
Which persona is this feature for?
System Operator

Exit Criteria

New tests are added that verify that by changing the template, the used topic are different from the default one.

Time Estimate (optional):
How many developer-days do you think this may take to resolve?
2

Additional context (optional)

/area broker
/area channel
/area control-plane

[matzew]

Each broker installation could than have their own topics, in a centralized kafka system

[Cali0707]

/assign