[Refactor] [history server] Rename cluster session key of events

[JiangJiaWei1103]

For now, we use the cluster session key as the identifier to isolate different sessions within the same Ray cluster. However, the "clusterName" key name of events should be renamed to "clusterSessionKey" [1] for better readability and maintainability.

Related PR

#4464

References

[1] https://github.com/Future-Outlier/kuberay/blob/0843e1a50486e12323d09af1a07c012b9fe59422/historyserver/pkg/eventserver/eventserver.go#L153

[Future-Outlier]

thank you! given that this is not urgent, we can do this in the beta