Save IP local to decrease k8s API calls

[smuu]

https://github.com/celestiaorg/knuu/blob/287c841e1720f5342bad4f1e09145e4c3fcbf11f/pkg/knuu/instance.go#L549

The user might call GetIP on an instance multiple times, which results in many unnecessary k8s API calls. This can be avoided by saving the IP as an Instance variable after it was requested the first time and then returned if requested again.