support kotlin generation in the maven plugin

[codefromthecrypt]

In Zipkin, we are starting to use Wire, initially in tests, but perhaps more places over time. It would be nice to be able to generate Kotlin as we are rewriting our server code in Kotlin.

We would like to be able to do this even if we use Maven as our build tool.

[swankjesse]

I’d like to do this, but not necessarily hold back 3.0 until this is ready.