KAFKA-6670: Implement a Scala wrapper library for Kafka Streams

[debasishg]

This PR implements a Scala wrapper library for Kafka Streams. The library is implemented as a project under streams, namely :streams:streams-scala. The PR contains the following:

• the library implementation of the wrapper abstractions
• the test suite
• the changes in build.gradle to build the library jar

The library has been tested running the tests as follows:

$ ./gradlew -Dtest.single=StreamToTableJoinScalaIntegrationTestImplicitSerdes streams:streams-scala:test
$ ./gradlew -Dtest.single=StreamToTableJoinScalaIntegrationTestImplicitSerdesWithAvro streams:streams-scala:test
$ ./gradlew -Dtest.single=WordCountTest streams:streams-scala:test

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)