Add GraphQL subscription for health status updates

What does this MR do and why?

Issue: #385717 (closed)

This adds a subscription issuableHealthStatusUpdated that triggers once the health status of an Issue or WorkItem gets updated.

Unfortunately we can't test subscriptions in graphiQL, but you can have a look at the frontend MR where the subscription gets used: !108853 (merged).

Screenshots or screen recordings

Once the frontend is rolled out:

health_status_subscription

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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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