Override retail_v2::UserEventServiceClient Authentication Defaults

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  [](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::retail_v2::UserEventServiceClient(
        google
::cloud::retail_v2::MakeUserEventServiceConnection(options));
 
}