Secure Suggested Reviewer gRPC with TLS

What does this MR do and why?

This MR ensures the gRPC channel between GitLab and Suggested Reviewer is encrypted with server-TLS.

The user-facing functions are gated behind feature flag suggested_reviewers

Relates to https://gitlab.com/gitlab-org/modelops/applied-ml/review-recommender/recommender-bot-service/-/issues/19

How to set up and validate locally

1. Run the recommender-bot-service in a development K8s cluster
2. Update the rpc_url value, e.g suggested-reviewer.dev:443
3. Copy the certificate used by the service in Step 1 to the gitlab folder, e.g. dev.crt
4. In the gitlab folder, try bundle exec rails c

   pry(main)> certs = File.read('dev.crt')
   pry(main)> c = ::Gitlab::AppliedMl::SuggestedReviewers::Client.new(rpc_url: 'suggested-reviewer.dev:443', certs: certs)
   pry(main)> d = c.suggested_reviewers(project_id: 15238759, merge_request_iid: 1, top_n: 5, changes: ['db', 'ee/db'], author_username: 'joe')
   => {:version=>"0.1.0", :top_n=>5, :reviewers=>[]}

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.