Integer is preferable for the type of observedGeneration

[dulltz]

At the v0.10.0 release, observedGeneration became to contain a numeric value,
but the type of the field is still String.

$ kubectl get rollout -n default myapp -o json | jq .status.observedGeneration
"2"

Is the Argo-rollout community planning to convert the type to Integer?
In my opinion, we should do it because some tools in the eco-system expect the type of observedGeneration is Integer.

For example, kstatus accepts the Integer observedGeneration but does not accept numeric String. https://github.com/kubernetes-sigs/cli-utils/blob/dfab5b40aa3d8cf8493abcf4aad46c177fa0a44c/pkg/kstatus/status/generic.go#L83

[gecube]

#3402