loadIdentity

[Focshole]

Hi,
As first, thank you for making this project. It is very useful and it looks well written.
I have noticed that there is a dht::crypto::saveIdentity function but there is no dht::crypto::loadIdentity function. I was wondering if it can be added, so that the identity can be preserved through multiple dht::runner runs.

Just in case, I have already written the code to do so:

Identity loadIdentity(const std::string &path,
                        const std::string &privkey_password = "") {
  std::ifstream pkStream(path + ".pem", std::ios::in | std::ios::binary);
  std::vector<uint8_t> pkContent((std::istreambuf_iterator<char>(pkStream)),
                                 std::istreambuf_iterator<char>());
  auto key = std::make_shared<PrivateKey>(pkContent, privkey_password);
  pkStream.close();
  // Create a certificate
  gnutls_x509_crt_t gnuCert;
  if (gnutls_x509_crt_init(&gnuCert) != GNUTLS_E_SUCCESS)
    throw std::runtime_error("Failed to initialize gnutls certificate struct");
  gnutls_datum_t crtContent;
  // Read the certificate file
  gnutls_load_file((path + ".crt").c_str(), &crtContent);
  gnutls_x509_crt_import(gnuCert, &crtContent, GNUTLS_X509_FMT_PEM);
  auto cert = std::make_shared<Certificate>(gnuCert);
  return {std::move(key), std::move(cert)};
}

Feel free to review/edit/import it as you wish!