GH-3213: Expose LeaderInitiatorFB.candidate

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

src/reference/asciidoc/whats-new.adoc

@@ -110,3 +110,9 @@ See <<./ip.adoc#failover-cf,TCP Failover Client Connection Factory>> for more in
 
 A `decodeFluxAsUnit` option has been added to the `RSocketInboundGateway` with the meaning to decode incoming `Flux` as a single unit or apply decoding for each event in it.
 See <<./rsocket.adoc#rsocket-inbound,RSocket Inbound Gateway>> for more information.
+
+[[x5.3-zookeeper]]
+=== Zookeeper Changes
+
+A `LeaderInitiatorFactoryBean` (as well as its XML `<int-zk:leader-listener>`) exposes a `candidate` option for more control over a `Candidate` configuration.
+See <<./zookeeper.adoc#zk-leadership,Leadership event handling>> for more information.

src/reference/asciidoc/zookeeper.adoc

@@ -118,3 +118,6 @@ public LeaderInitiatorFactoryBean leaderInitiator(CuratorFramework client) {
 }
 ----
 ====
+
+Starting with version 5.3, a `candidate` option is exposed on the `LeaderInitiatorFactoryBean` for more configuration control of the externally provided `Candidate` instance.
+Only one of the `candidate` or `role` options has to be provided, but not both; the `role` options creates internally a `DefaultCandidate` instance with an `UUID` for `id` option.