Fixes https://github.com/spring-projects/spring-integration/issues/3213 * Expose a `LeaderInitiatorFactoryBean.setCandidate()` for better control over candidate options * Expose a `candidate` attribute for the `<int-zk:leader-listener>` tag * Fix `dokka` task to obtain `package-list` URL from the local dir * Migrate ZK tests to JUnit 5