VSA: Capture issue assignment events
Problem
We current can't measure the time taken from an issue being created, to assigned for the first time.
Proposal
The table could follow the structure of
resource_label_events
:resource_assignment_events
- action (0: assignment, 1: unassignment). Note: we don't really need unassignment at the moment
- issue_id
- merge_request_id
- user_id
- created_at
When adding or removing assignees, we should create a new record in the DB.
Copied from #361983 (comment 1318526653)