debug/generate_test_user_sig library

Module: GenerateTestUserSig

Description: generates UserSig for testing. UserSig is a security signature designed by Tencent Cloud for its cloud services. It is calculated based on SDKAppID, UserID, and EXPIRETIME using the HMAC-SHA256 encryption algorithm.

Attention: do not use the code below in your commercial app. This is because:

       The code may be able to calculate UserSig correctly, but it is only for quick testing of the SDK’s basic features, not for commercial apps.
       `SECRETKEY` in client code can be easily decompiled and reversed, especially on web.
        Once your key is disclosed, attackers will be able to steal your Tencent Cloud traffic.

       The correct method is to deploy the `UserSig` calculation code and encryption key on your project server so that your app can request from your server a `UserSig` that is calculated whenever one is needed.
      Given that it is more difficult to hack a server than a client app, server-end calculation can better protect your key.

Reference: https://cloud.tencent.com/document/product/647/17275#Server