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))));
 
}