Kotlin implementation

Kotlin gRPC exposes a different server endpoint API from Java. It would be useful to have an example of throwing a StatusException, if that's what the Kotlin generated code expects users to do.

With this in `build.gradle`:

```
plugins {
id "com.google.protobuf" version "0.8.12"
}
protobuf {
protoc { artifact = "com.google.protobuf:protoc:3.12.2" }
plugins {
grpc { artifact = "io.grpc:protoc-gen-grpc-java:1.29.0" }
grpckt { artifact = "io.grpc:protoc-gen-grpc-kotlin:0.1.2" }
}
generateProtoTasks {
all()*.plugins {
grpc {}
grpckt {}
}
}
}
```

The generated RPC functions to override are like this:

```kotlin
/**
* Holder for Kotlin coroutine-based client and server APIs for
* my_pkg.MyService.
*/
object MyServiceGrpcKt {
...

/**
* Returns the response to an RPC for valohealth_monocle.model_search.ModelSearchService.Search.
*
* If this method fails with a [StatusException], the RPC will fail with the corresponding
* [io.grpc.Status]. If this method fails with a [java.util.concurrent.CancellationException],
* the RPC will fail
* with status `Status.CANCELLED`. If this method fails for any other reason, the RPC will
* fail with `Status.UNKNOWN` with the exception as a cause.
*
* @param request The request from the client.
*/
open suspend fun search(request: ModelSearchRequest): ModelSearchResponse = throw
StatusException(UNIMPLEMENTED.withDescription("Method my_pkg.MyService.Search is unimplemented"))
}
```

So unlike Java, there's no `observer` to tag errors onto, but the default implementation here seems to give an example of throwing an error.