docs: public static String encodeAppSecretProof(String appSecret, String accessToken) {
      try {
        byte[] key = appSecret.getBytes(StandardCharsets.UTF_8);
        SecretKeySpec signingKey = new SecretKeySpec(key, "HmacSHA256");
        Mac mac = Mac.getInstance("HmacSHA256");
        mac.init(signingKey);
        byte[] raw = mac.doFinal(accessToken.getBytes());
        byte[] hex = encodeHex(raw);
        return new String(hex, StandardCharsets.UTF_8);
      } catch (Exception e) {
        throw new IllegalStateException("Creation of appsecret_proof has failed", e);
      }
    }