Replace ListenableFuture with CompletionStage / CompletableFuture

[rulle-io]

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, the same request is mentioned there.

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<ResponseType> unaryExample(RequestType request)
+public CompletionStage<ResponseType> unaryExample(RequestType request)