[lucliu1108]

Summary

Cluster-wide broker configs and listener-prefixed broker configs reach
createBrokerConfigEntry as raw strings instead of parsed values. For
LIST configs, passing them to ConfigDef.convertToString would throw
ClassCastException, so a guard returns them unchanged. No test covered
this guard yet.

This PR adds two tests to DescribeConfigsTest:

1. testDescribeConfigsClusterWideListConfig: sets
   log.cleanup.policy=compact,delete as a cluster-wide dynamic config and
   describes it.
2. testDescribeConfigsPrefixedListenerListConfig: sets
   listener.name.external.ssl.enabled.protocols as a static broker config
   and describes it.