Add securityPoliciesSyncUpdated GraphQL subscription

What does this MR do and why?

We are working on visualising the synchronisation status of merge request approval policies in the policy overview UI (#559273 (closed)).

This MR introduces the Redis state and exposes callbacks that will be called from services/workers in !201782 (merged).

References

• &19049
• #559273 (closed)

Screenshots or screen recordings

• #559273 (comment 2684732955)

How to set up and validate locally

Updates can't be tested until !201782 (merged) was merged.

subscription {
  securityPoliciesSyncUpdated(policyConfigurationId: "gid://gitlab/Security::OrchestrationPolicyConfiguration/114") {
    projectsProgress
    projectsTotal
    failedProjects
    mergeRequestsProgress
    mergeRequestsTotal
  }
}

MR acceptance checklist

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Related to #528300 (closed)