Add SystemConfig Legacy Handler

- Add SystemConfi Legacy handler
- Add service configuration to specify which configs are exposed to the frontend
- Update Systemconfig Item and Collection data provider to use legacy handler
- Add unit tests for legacy handler

config/services.yaml

@@ -20,6 +20,7 @@ services:
       $legacyDir: '%legacy.dir%'
       $legacySessionName: '%legacy.session_name%'
       $defaultSessionName: '%default_session_name%'
+      $exposedSystemConfigs: '%legacy.exposed_system_configs%'
 
   # makes classes in src/ available to be used as services
   # this creates a service per class whose id is the fully-qualified class name

config/services/legacy/exposed_system_configs.yaml

@@ -0,0 +1,6 @@
+parameters:
+    legacy.exposed_system_configs:
+        default_language: true
+        passwordsetting:
+            forgotpasswordON: true
+        languages: true