+clu #15365 Make cluster.UniqueAddress public

(cherry picked from commit 2a2caeebd585d6dde53bbca0125e62626f7bae51)

Conflicts:
	akka-cluster/src/main/scala/akka/cluster/Member.scala
Merge pull request #15382 from akka/wip-15365-UniqueAddress-master-patriknw

+clu #15365 Make cluster.UniqueAddress public (for validation)