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)