Ban method definitions with plain void as return type

[artem-v]

if (returnType.isAssignableFrom(Void.TYPE)) {
  throw new IllegalMethodDefinitionWeDontSupportThisException();
}

You can define:

  Mono<Void> funcVoidReactive();

You should not be allowed to define:

void funcNonReactive();

Motivation: it's hard to combine plain void functions, let's define convention that our service definitions has to be pure functions, i.e. must return somerhinhg , even if it's Mono. Why do that? For caring about api clients, about their ability to combine.

[artem-v]

looping @ronenhamias