Make DerivedState from Configs

[mlajkim]

Current SIA v3.2 turns on/off based on given configurations, but it has the following probles:

• Easy to make mistakes for developers
  • Let's say the following is the correct setting that makes SIA to read local instance cert
  • this.config.ProviderService === "" && this.config.CertFile === "" && this.config.KeyFile === ""
  • Developers might forget to include this.config.CertFile === "" && this.config.KeyFile === "" and only include this.config.ProviderService === ""
  • Actually happened and fixed in https://github.com/AthenZ/k8s-athenz-sia/pull/111/commits/27d35227eb6e7602950128fc62a10fa616a87abb
• Hard to read
  • The following code is so vague and has no meaning.
  • this.config.ProviderService === "" && this.config.CertFile === "" && this.config.KeyFile === ""
  • if we can have a DerivedState that holds such combination, it would be much easier to read

    DerivedState := func() {
    if this.config.ProviderService != "" {
    return "identityd"
    }
    if config.CertFile === "" &&config.KeyFile === ""  {
    return "readLocalFile"
    }
    return "inactive"
    }()