Download pekko-multi-node-testkit_3 JAR file with all dependencies

This module of Apache Pekko is marked as 'may change', which means that it is in early access mode. A module marked 'may change' doesn't have to obey the rule of staying binary compatible between minor releases. Breaking API changes may be introduced in minor releases without notice as we refine and simplify based on your feedback. Additionally such a module may be dropped in major releases without prior deprecation.