Override @c BlockingPublisher Default Authentication

 namespace pubsub = ::google::cloud::pubsub; using ::google::cloud::Options; [](std::string const& keyfile) { auto is = std::ifstream(keyfile); is.exceptions(std::ios::badbit); auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {}); return pubsub::BlockingPublisher(pubsub::MakeBlockingPublisherConnection( Options{}.set<google::cloud::UnifiedCredentialsOption>( google::cloud::MakeServiceAccountCredentials(contents)))); }