HPA support for graphd

[xiajingchun]

From customer:

Theortically Graphd is a stateless service. But in k8s setup it is still desgined as statefulset… So ideally this can be migrated to Deployment rather than statefulset. But this can be done as a separate task.

The ask here is to enable HPA for graphd… I don’t see it currently being supported in nebula operator.

https://github.com/vesoft-inc/nebula-operator/blob/v1.5.0/charts/nebula-cluster/templates/nebula-cluster.yaml#L10C51-L10C51

[wenhaocs]

Please consider scaling with CPU resource only for the first release.

[MegaByte875]

#302