Use `spotless` to check the style of Scala code

[Chillax-0v0]

Currently, we are only performing style checks on Scala files under the 'streams' directory:

// build.gradle
spotless {
  scala {
    target 'streams/**/*.scala'
    scalafmt("$versions.scalafmt").configFile('checkstyle/.scalafmt.conf').scalaMajorVersion(versions.baseScala)
    licenseHeaderFile 'checkstyle/java.header', 'package'
  }
}

Once the project stabilizes, we can perform similar checks on the Scala files we have modified under the 'core' directory.

[Chillax-0v0]

As we aim to make as few modifications as possible to the original Kafka code, we do not add additional code style checks.