API: New /users/:id/events endpoint

What does this MR do?

If add a new /users/:id/events endpoint to retrieve a user's contribution events. The events returned are filtered so that only the events for projects that the current user can see are returned (similarly to what we do at the controller level).

Why was this MR needed?

Because it's a nice feature to calculate leaderboards, for instance for #17815 (closed).

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #20866 (closed).