Create common super-interface for containers

[see-quick]

As discussed strimzi/strimzi-kafka-operator#6400, we should create a common super-interface for StrimziKafkaCluster and StrimziKafkaContainer to allow precisely this kind of abstraction in users. The reason why we need it is that currently, we can't use super-type of StrimziKafkaContainer, StrimziZooKeeperContainer...and the only supertype is Startable. This interface should expose getBootstrapServers() and getZookeeperConnectString() to the users.

✅ It would also be more typesafe, since trying to call this method with any old Startable would then be a compile-time error.