Implement mergeRequestReviewersUpdated subscription
What does this MR do and why?
Introduces a new mergeRequestReviewersUpdated
GrpahQL subscription that can be subscribed to by the frontend to get real-time updates on reviewers whenever they get updated.
This only includes the subscription and the triggers will be done separately (#366857 (closed) and #366858 (closed)).
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #366856 (closed)