Override the authentication configuration

  namespace bigtable = ::google::cloud::bigtable;
  [](std::string const& project_id, std::string const& instance_id,
     std::string const& table_id, std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto options =
        google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents));
    auto resource = bigtable::TableResource(project_id, instance_id, table_id);
    return bigtable::Table(bigtable::MakeDataConnection(options), resource);
  }