Send review request email to new reviewers

What does this MR do and why?

We've added diff summary to review re-request email in !127339 (merged), but we also need to send something for initial review request. We currently don't send a specific review request email to reviewers, but we're sending a generic reviewer change email that we're sending it to every participants.

This MR sends an additional specific review request email to new reviewers. They will receive two emails for now and we may need to tweak it to avoid sending two emails though.

Screenshots or screen recordings

This will send the same email as the review re-request email.

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. Visit http://127.0.0.1:3000/admin/sidekiq/scheduled and see new delayed mail has been scheduled.
6. 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)