Implement mergeRequestMergeStatusUpdated subscription
What does this MR do and why?
Introduces a new mergeRequestMergeStatusUpdated
GraphQL subscription that can be subscribed to by the frontend to get real-time updates on merge status whenever it gets updated.
This only includes the subscription and the triggers will be done separately.
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 #372514 (closed)