feat: creat course clone tasks completed signals

Adds a signal when all course clone tasks finish

Testing Instructions

0. clone ytp_api and checkout the branch from this PR

1. Clone the demo course with the following command

curl --request POST \
  --url http://studio.local.overhang.io:8001/clone_course/clone/ \
  --header 'Authorization: Bearer <ACCESS TOKEN>' \
  --header 'Content-Type: application/json' \
  --data '{
    "source_id": "course-v1:OpenedX+DemoX+DemoCourse",
    "dest_id": "course-v1:OpenedX+DemoX+DemoCourse_CLONE",
    "start_date": "2023-01-01T00:00:00Z",
    "overview": "This is a cloned course."
}
'

2. You can get the access token at the folowing URL: http://apps.local.overhang.io:8000/admin/oauth2_provider/accesstoken/
3. Make sure the cms, cms-workers and lms logs don't throw any errors other than not being able to call the ASU endpoint

Private-Ref: https://tasks.opencraft.com/browse/BB-9001