Kotlin support for Spark interpreter

This issue aims to add Kotlin language support in Apache Zeppelin, adding a standalone interpreter as well as an interpreter for Spark group.

Summary:

• Features:
  • Kotlin 1.3.50 support in standalone %kotlin interpreter
  • Kotlin with Spark 2.4.x support in Spark interpreter, available under %spark.kotlin (Kotlin uses Java Spark API)
  • ZeppelinContext support in Spark interpreter that allows visualisations, object sharing, etc.
  • A bound KotlinContext that provides access to bound and user-defined variables and functions. Example:

    val x = listOf(1, 2, 3);

    kc.showVars()

    will print

    kc: KotlinContext! = org.apache.zeppelin.kotlin.repl.KotlinRepl$KotlinContext@2ae76226
    x: List<Int> = [1, 2, 3]

• Implementation details:
  • Kotlin interpreter uses Kotlin REPL tools to interpret code.
  • Variable and function binding is done via implicit receiver object passed to REPL, so user code is executed inside Kotlin's with block, making receiver's fields and methods visible in the scope.
  • REPL generated classes can be written to spark.repl.classes.outputDir, making them available in Spark jobs.
• Licensing
  • All Kotlin libraries have Apache 2.0 license.
• Documentation
  • Kotlin documentation is added to ./docs/interpreter, and Spark documentation is updated
• Testing
  • Kotlin interpreter has unit tests in its module, and Kotlin Spark interpreter tests are added.