sign(timestamp, secretKey) {
        return Crypto
            .createHmac("sha256", Buffer.from(secretKey, "utf8"))
            .update(Buffer.from(`${timestamp}\n${secretKey}`, "utf8"))
            .digest("base64");
    }
DingDing sign
@param {Date} timestamp Timestamp of message
@param {string} secretKey Secret key to sign data with
@returns {string} Base64 encoded signature