Align Akka gRPC examples

[octonato]

The Akka gRPC docs have a protobuf definition with 4 methods: unary call, client stream, server stream and bi-directional.

The quickstart for Akka has only two methods and the Play and Lagom only one method. We should use the same protobuf definition all over the place, in both Java and Scala.

There are a few advantages for that:

• users will read the docs and see the same example on the quickstart
• if all quickstart samples use the same protobuf, it become easier to maintain that project: https://github.com/lightbend/akka-grpc-hands-on

Akka gRPC Hands-on has an extracted (and currently adapted) version of each quickstart.

[ignasi35]

see

• Update hello.proto to demo streaming calls play-scala-grpc-example#23
• Update hello.proto to demo streaming calls play-java-grpc-example#32