Implement pipeline hooks, extracted from !5525

What does this MR do?

Implement pipeline hook upon:

• When Pipeline is created,
• When a pipeline status changes,
• When a pipeline finished (this would basically be done by 2.).

What are the relevant issue numbers?

Closes #20115 (closed)

Screenshot

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug

This is rebased on top of which simplifies a pipeline processing: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5782

/cc @ayufan