This step extends the generated server skeleton code to write a simple server
that provides the hello service. This in introduces two new classes
that provides the hello service. This introduces two new classes:
- a service implementation [GreetingsImpl.java](src/main/java/ex/grpc/GreetingsImpl.java).
- a server that hosts the service implementation and allows to accessed over the network: [GreetingsServer.java](src/main/java/ex/grpc/GreetingsServer.java).
- a server that hosts the service implementation and allows access over the network: [GreetingsServer.java](src/main/java/ex/grpc/GreetingsServer.java).