Support Kafka and Spark plugins with Scala 2.12

Current kafka and spark plugins are using Scala 2.11, which will have conflict if other users want to build their own plugins using Scala 2.12, so we should try to publish both libs and allow users to pick which libs to be included during startup.

The issue names no files or tests; first locate the Kafka and Spark plugin build definitions and their startup loading entry points. Check how Scala 2.11 artifacts are published and selected, then verify that users can choose compatible Scala 2.11 or 2.12 libraries without conflicts.

Written by the indexing model from the issue text.