[Feature] Add support for Kotlin

[bkrichards]

Kotlin data classes are great but default parameters quickly diminish the ability for the object to be consumed by Java in an idiomatic fashion.

data class MyObject(val name: String, val age: Int = 0)

class MyObjectBuilder() {
    private lateinit var name: String
    private var age: Int = 0

    fun withName(val name: String) = this.name = name
    fun withAge(val age: Int) = this.age = age
    fun build() = MyObject(name, age)
 }

For non-nullable fields could store them as null in the builder with added validation or just rely on lateinit validation to throw when the property is read during build.