Include review summary in new review email

What does this MR do and why?

When a review is created we send an email to notify users and we want to include the review summary generated by AI.

To do that, we delay the sending of the new_review_email by 2 minutes as we make a request to AI to generate the review summary asynchronously.

If after 2 minutes the review summary is still not available for a specific review, then the new review email will be sent without it.

We are still generating review summary behind automatically_summarize_mr_review feature flag.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Enable automatically_summarize_mr_review feature flag.
2. Follow https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally.
3. Submit a review on a MR not authored by you.
4. Wait for 2 minutes and visit http://localhost:3000/rails/letter_opener and a review email should be there with the review summary.

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 #417219 (closed)