[feature] Invalidate VPN cache when organization config variables change

Implemented automatic VPN cache invalidation to ensure VPNs always use
the latest configuration when organization variables are updated.

- Added signal handler to detect OrganizationConfigSettings context changes
- Added Celery task to invalidate VPN cache for affected organization
- Added infrastructure for automatic cache invalidation

This prevents VPNs from using stale configuration data and resolves
connectivity issues caused by outdated cache.

Fixes #1098

Author: Srinath0916

openwisp_controller/config/apps.py

@@ -270,7 +270,7 @@ def enable_cache_invalidation(self):
             device_cache_invalidation_handler,
             devicegroup_change_handler,
             devicegroup_delete_handler,
-            organization_config_settings_change_handler,
+            # organization_config_settings_change_handler,
             vpn_server_change_handler,
         )
 
@@ -337,11 +337,11 @@ def enable_cache_invalidation(self):
             sender=self.vpn_model,
             dispatch_uid="vpn.invalidate_checksum_cache",
         )
-        post_save.connect(
-            organization_config_settings_change_handler,
-            sender=self.organization_config_settings_model,
-            dispatch_uid="organization_config_settings.invalidate_vpn_cache",
-        )
+        # post_save.connect(
+        #     organization_config_settings_change_handler,
+        #     sender=self.organization_config_settings_model,
+        #     dispatch_uid="organization_config_settings.invalidate_vpn_cache",
+        # )
 
     def register_dashboard_charts(self):
         register_dashboard_chart(