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.

## What are the relevant issue numbers?

Closes #20866.

See merge request !6771