Add diff summary to request review email

What does this MR do and why?

This MR adds diff summary generated by AI to re-review request emails. There will be a follow up MR to add similar email for newly assigned reviews as well.

Screenshots or screen recordings

HTML version

Text version

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. Create a test MR with some changes.
4. Assign a reviewer.
5. Leave a note while logged in as a reviewer in another browser session.
6. Request re-review as the author by clicking the re-review button next to the reviewer handle.
7. Visit http://127.0.0.1:3000/admin/sidekiq/scheduled and see new delayed mail has been scheduled.
8. Visit http://localhost:3000/rails/letter_opener once 2 minutes has pass and check the received email. -->

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 #414401 (closed)