Kotlin: add Gradle task to build a "fat" jar

[tjbanghart]

I'd like to include the Kotlin SDK in another JVM project. The current jar task only builds the SDK but doesn't include the ktor, gson, or general Kotlin specific dependencies. A dependent project would need to include these separately.

Adding the Shadow plugin will let us build shaded "fat" jars that include all dependencies so developers could build a single jar to bundle.