Allow asynchronous functions in an interceptor

[birdthedeveloper]

In order to send a twirp RPC call, in my use case, I need to get an authentication token via asynchronous method first. It seems to me that this cannot be done with the current implementation.

Desired code, most of which taken from official interceptor example in documentation:

// This does not compile

tart.Interceptor LogInterceptor(Logger l) {
  return (Method next) {
    return (Context ctx, dynamic req) async {

      // added this line bellow
      String token = await getAuthenticationToken();

      l.print('''Service: ${ctx.value(tart.ContextKeys.serviceName)}, Method: ${ctx.value(tart.ContextKeys.methodName)}''');
      return next(ctx, req);
    };
  };
}