New pluginapi method for syncables (#30790)

* new pluginapi method for syncables
---------
Co-authored-by: Mattermost Build <build@mattermost.com>

server/i18n/en.json

@@ -5358,10 +5358,18 @@
     "id": "app.get_user_team_scheduled_posts.error",
     "translation": "Error occurred fetching scheduled posts."
   },
+  {
+    "id": "app.group.create_syncable_memberships.error",
+    "translation": "Unable to create group syncable memberships."
+  },
   {
     "id": "app.group.crud_permission",
     "translation": "Unable to perform operation for that source type."
   },
+  {
+    "id": "app.group.delete_invalid_syncable_memberships.error",
+    "translation": "Unable to delete invalid group syncable memberships."
+  },
   {
     "id": "app.group.group_syncable_already_deleted",
     "translation": "group syncable was already deleted"