Suggestion: Add GRPC status codes as constants

[dominiks]

While working with server side Nakama RPCs I found myself using the suggested grpc status codes and I added them to a constants file as they were not available anywhere before.

I suggest adding them to the NakamaException or alternatively to the Nakama autoload for easy access when working with gRPC. A named enum GRPC could also be used instead of prefixing it to all constants but this is the most 'Godot-way', I think.

Additionally there is the field status_code on NakamaException. I found it somewhat confusing in the beginning as it is not clear from the documentation whether that is http-status code or Godot errors. In Godot the http codes are available as RESPONSE_* constants - would it make sense to rename the property to response_code?