Sync with local json task file

[JulienBrn]

Is your feature request related to a problem? Please describe.
I'm currently running label studio with a local storage source for keypoint labeling. I need to have additional metadata about the images that are being labeled, so I'm using the json format for tasks and I have switched off "Treat every bucket object as a source file" in my source storage.
The json file describing the tasks is always stored at the same specific path (that I could transform into a URL if necessary). Is there anyway to keep the annotations tasks and the json file in sync (when pressing the sync button in the UI) ?

Describe the solution you'd like
In source storage, be able to pass the path to a json file containing tasks. The sync button should update the annotation tasks using that json file (creating new ones, updating the metadata of old ones and perhaps deleting tasks). The merge should be done using the id field.
Buttons in the UI that modify the task (such as the delete task) should either not be usable in this case or should update the json file depending on some source storage configuration.