feat: (c sshnpd) rejection notification

[realvarx]

- What I did

• Created_response_atkey and notify methods for code reuse
• Signature verification rejection notification

- How I did it

- How to verify it
If we intentionally modify the signature value to make it incorrect, we can see what it happens:

value[0] = 'A';

  res = verify_envelope_signature(requesting_atsign_publickey, (const unsigned char *)payloadstr,
                                  (unsigned char *)value, hashing_algo_str, signing_algo_str);
  if (res != 0) {

    // Notify noports client that this session is NOT connected
    memset(value, 0, valuelen);
    snprintf(value, valuelen, "Signature verification failed: %d\n", res);
    atclient_atkey error_res_atkey;
    atclient_atkey_init(&error_res_atkey);

    create_response_atkey(&error_res_atkey, atsign, requesting_atsign,
                                 identifier, keyname, &keynamelen);

    notify(atclient, atclient_lock, &error_res_atkey, value);
    atclient_atkey_free(&error_res_atkey);

    atlogger_log(LOGGER_TAG, ATLOGGER_LOGGING_LEVEL_ERROR, "Failed to verify envelope signature\n");
    free(envelope);
    atchops_rsakey_publickey_free(&requesting_atsign_publickey);

    return;
  }

- Description for the changelog

Closes #1117