Add first assigned to Issue and MR events to VSA

What does this MR do and why?

This MR adds two stage events to value stream analytics:

• Issue first assigned
• Merge request first assigned

Database

It's not straightforward to extract the underlying query, we'll have issue or MR ids where we do an extra join:

• https://explain.depesz.com/s/3TBX (uncached, reads maximum 200m rows)
• https://explain.depesz.com/s/FVVy (uncached, reads maximum 200 rows)

How to set up and validate locally

Value stream configuration:

1. Ensure you're on ultimate or on premium
2. Enable the FF: record_issue_and_mr_assignee_events (should be enabled by default now)

   Feature.enable(:record_issue_and_mr_assignee_events)

3. Go to a group > analytics > value stream
4. Create a new value stream based on the image above
5. Create an MR and an issue (if your group doesn't have any)
6. Wait a few seconds
7. Assign them to any user
8. Invoke the data aggregation manually:

   Analytics::CycleAnalytics::IncrementalWorker.new.perform

9. Go back to the value stream page, some figures should show up on the page.

Data shows up:

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #361983 (closed)