Refactor audit events for Repository

What does this MR do and why?

Adds an audit event type for repository_download_operation. Refactors associated API helpers to use Gitlab::Audit::Auditor to build audit events.

NOTE: While repository_download_operation does not follow the preferred noun_verb format of new audit events, this event name was already being used in ee/app/controllers/ee/projects/repositories_controller.rb. It is reused here to prevent duplication and compatibility issues.

MR acceptance checklist

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Related to #374108 (closed)