Add secret for GitLab Suggested Reviewers

What does this MR do?

As part of https://gitlab.com/gitlab-org/gitlab/-/issues/379635+, we've decided to add a new GitLab internal endpoint for Suggested Reviewers. For more context, please see the related issue.

Suggested Reviewers will use this shared secret to sign a time-based JWT token which is then validated on the GitLab side.

This MR takes care of generating and symlinking this shared secret.

Related issues

https://gitlab.com/gitlab-org/gitlab/-/issues/379635

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened (gitlab-org/charts/gitlab!2875 (merged))