Replace ListenableFuture with CompletionStage / CompletableFuture

Thanks for stopping by to let us know something could be better!

**Is your feature request related to a problem? Please describe.**
E.g. [#2688](https://github.com/grpc/grpc-java/issues/2688), the same request is mentioned [there](https://github.com/grpc/grpc-java/issues/2688#issuecomment-386447024).

**Describe the solution you'd like**
To add code generation configuration parameter to use CompletionStage instead of ListenableFuture.

From https://grpc.io/docs/languages/java/generated-code/#unary-3 :
```
Unary

The future stub signature for a unary RPC method unaryExample:

-public ListenableFuture unaryExample(RequestType request)
+public CompletionStage unaryExample(RequestType request)
```