Override appengine_v1::DomainMappingsClient Authentication Defaults

  [](std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);  // Minimal error handling in examples
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto options =
        google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents));
    return google::cloud::appengine_v1::DomainMappingsClient(
        google::cloud::appengine_v1::MakeDomainMappingsConnection(options));
  }