Support scalar types in codegen

[pvlugter]

Should we allow specifying domain types as scalar value types, rather than always being a message type? Probably most relevant to replicated entities, where something like string keys for maps may be common. This could be specified using the protobuf scalar value types, and then translated to an appropriate type in the language SDK.

So where it's currently:

option (akkaserverless.file).replicated_entity = {
  name: "SomeCounterMap"
  entity_type: "some-counter-map"
  replicated_counter_map: {
    key: "SomeKey"
  }
};

message SomeKey {
  string key = 1;
}

It could instead be:

option (akkaserverless.file).replicated_entity = {
  name: "SomeCounterMap"
  entity_type: "some-counter-map"
  replicated_counter_map: {
    key: "string"
  }
};

[pvlugter]

I'll follow up with this for the Replicated Entity codegen.