Support both KRaft combined mode and separate controllers

[tombentley]

Currently the extension supports ZooKeeper mode (using @ZooKeeperCluster as the constraint annotation) and KRaft in combined mode (using @KraftCluster). It would be good to also support KRaft using separate controllers.

This could be expressed in the API in several ways. Perhaps the simplest is just to add a combinedMode attribute to @KraftCluster.

Although the API is simple the knock-on consequences are not.

• KafkaClusterConfig has a kraftMode boolean flag that is perhaps better treated as a 3 element MetadataMode enum (of ZK, Kraft combined and Kraft separate).
• KafkaClusterConfig has a getBrokerConfigs() that would now be more clearly called getNodeConfigs(), since not all nodes are brokers.
• ConfigHolder has getBrokerNum() that would now be more clearly called getNodeId(), again because not all nodes are brokers.
• KafkaCluster.getNumOfBrokers() could remain a count of the brokers (not of all the nodes), but it might also be useful to have a count of all nodes.
• The test coverage, which already has to cope with (ZK, combined) × (in-VM, containers) would need to also cover the separate controller cases.

See discussion in #174 which led to this issue.