🔧 Create a configuration file interface

This commit adds the ability to provide a config file in different ways:

The following options are given in the order in which they are checked:
1. A config file path passed by the -c parameter on the command line
2. A config file path passed by the HS_CONFIGFILE environment variable
3. A config.json file in the application root

Should these fail we try to get the parameters for the config struct
directly from environment variables or secret files as used in a
container environment. As example the JwtKey can be supplied by either
passing the value through:

* An environment variable HS_CONFIG_JWTKEY which contains the value directly
* An environment variable HS_CONFIG_JWTKEY_FILE which contains a path
  pointing to the Secret File

middleware/middleware.go

@@ -23,7 +23,7 @@ func Authenticate() gin.HandlerFunc {
 			if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
 				return nil, fmt.Errorf("unexpected signing method")
 			}
-			return config.JwtKey, nil
+			return config.GlobalConfig.JwtKey, nil
 		})
 
 		if err != nil {