Secret config

[zero88]

Description

• Add secret part in NubeConfig (SecretConfig similar to AppConfig)
• Provide reference variable like: @<part>.<var_name> and in "__secret__" has @secret.<var_name> key. For example:

  {
    "__app__": {
      "password": "@secret.abc"
    },
    "__secret__": {
      "@abc": "<actual_value>"
    }
  }

DOD

• If ref_var not found, just return and keep it as it is
• Exclude secret config in logging
• Work well System properties and Environment variables