Add an 'is-in-service' health check wrapping rabbit:is_serving/0

[the-mikedavis]

Proposed Changes

This is useful for a load balancer, for example, to be able to avoid sending new connections to a node which is running and has listeners bound to TCP ports but is being drained for maintenance.

This PR covers the first part of #13782: adding a new health-check GET /api/health/checks/is-in-service.

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