List the update definitions available for a cluster that is a registered component of the
specified system. The cluster must be a component of the system; if it is not, the request
returns 404.

This is a read-only operation. The updateDefinitions array may be empty when no updates have
been synced for the cluster's type — an empty list is a normal result, not an error.

See also the non-system-scoped GET /api/clusters/{id}/available-updates (Clusters tag), which
lists the same update definitions for a cluster without requiring system membership. This
system-scoped variant additionally requires the cluster to be a registered component of the
named system.