Cleaner object creation

In the [sample code](https://github.com/GoogleCloudPlatform/kotlin-samples/blob/master/run/grpc-hello-world-streaming/src/main/kotlin/io/grpc/examples/helloworld/HelloWorldClient.kt) you initialize a HelloRequest object with:
```
val request = HelloRequest.newBuilder().setName(name).build()
```

It would be a big help if you could generate a Kotlin builder for each user protobuf object:
```
fun helloRequest(block: HelloRequest.Builder.() -> Unit): HelloRequest =
HelloRequest.newBuilder().let {
block.invoke(it)
it.build()
}
```

which would allow for much cleaner object creation:
```
val request = helloRequest { this.name = name }
```

[One of your fellow Googlers](https://github.com/googleapis/gapic-generator-kotlin) uses this approach and it makes for less cluttered user code.