Add audit events for GitLab Duo code review decisions

What does this MR do and why?

GitLab Duo code review can now approve a merge request or request changes on it, but these decisions leave no audit trail. Requested changes and their override only create system notes, and a GitLab Duo approval only streams the generic merge_request_approval_operation event. This merge request adds three audit event types, saved to the database and streamed:

• duo_code_review_approved and duo_code_review_changes_requested, authored by the GitLab Duo bot, when the approval or the request for changes succeeds. The duo_code_review_changes_requested event records whether override_requested_changes is already on, because the override stays set and GitLab Duo's new request then does not block the merge.
• duo_code_review_requested_changes_overridden, authored by the user, when a user overrides requested changes while GitLab Duo is one of the reviewers that requested them.

A GitLab Duo approval still streams merge_request_approval_operation as well, so existing streaming consumers that filter on that event keep seeing GitLab Duo approvals.

References

gitlab-org#23854

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