Suggested reviewers gRPC client

What does this MR do and why?

Suggested reviewers gRPC client for Applied Machine Learning

• The changes are behind a feature flag for user facing functions
• We shall have authentication and encryption setups on top of the current work
• This MR will unblock also the graphql, UI, the caching/service and background worker optimizations

Related to #368679 (closed) and https://gitlab.com/gitlab-org/modelops/applied-ml/review-recommender/recommender-bot-service/-/merge_requests/35

Screenshots or screen recordings

How to set up and validate locally

1. Run the recommender-bot-service
2. Update the rpc link
3. On bin/rails c try

c = Gitlab::AppliedMl::SuggestedReviewers::Client.new

d = c.suggested_reviewers(project_id: 15238759, merge_request_iid: 1, top_n: 5, changes: ['db', 'ee/db'], author_username: 'joe')

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 #368679 (closed)