Add auto_duo_code_review project setting

What does this MR do and why?

This MR adds Enable Automatic Duo Code Review setting to Project: Merge requests settings.

!187807 (merged) needs to be merged first.

References

#506537 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Follow these instructions to set up Duo features in your local environment.
2. Enable :ai_review_merge_request and :duo_code_review_chat FFs
3. Ensure you have a user with a developer role who has Duo Enterprise add-on
4. Visit Merge requests setting and check the newly added setting and save
5. Create a new MR with some changes
6. Observe GitLab Duo starting code review

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #506537 (closed)