Fix ServiceLoader issue with PluginClassLoader and add basic-auth-extension packaging & classpath
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Author: Magesh Nandakumar <magesh.n.kumar@gmail.com>
Reviewers: Konstantine Karantasis <konstantine@confluent.io>, Ewen Cheslack-Postava <ewen@confluent.io>
Closes#5135 from mageshn/KAFKA-6991