[MM-52924] Implement ConfigurationWillBeSaved plugin hook (#23567)

* Implement ConfigurationWillBeSaved plugin hook

* Add comment

* Update comment

* Fix potential nil dereference if plugin environment is unset

* Address PR review

server/i18n/en.json

@@ -6383,6 +6383,10 @@
     "id": "app.save_config.app_error",
     "translation": "An error occurred saving the configuration."
   },
+  {
+    "id": "app.save_config.plugin_hook_error",
+    "translation": "An error occurred running the plugin hook on configuration save."
+  },
   {
     "id": "app.scheme.delete.app_error",
     "translation": "Unable to delete this scheme."