Event type information in streaming audit event for project download

What does this MR do and why?

Adds event type repository_download_operation information in streaming audit event for zip download action for a repository.

How to set up and validate locally

1. Setup external destination using https://docs.gitlab.com/ee/administration/audit_event_streaming.html#use-the-gitlab-ui or https://docs.gitlab.com/ee/administration/audit_event_streaming.html#use-the-api
2. Download a project from UI.
3. Check streamed payload and headers contain event type repository_download_operation.
4. Check audit event is also visible under admin/audit_logs.
5. Logout.
6. Do steps 2-3 again for a public project.
7. Check streamed payload contains author_id: -1 and author_name: An unauthenticated user

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 #363086 (closed)