Implement update strategy where any change to the spec produces a new IstioRevision

We currently have two update strategies: InPlace and RevisionBased. The latter creates a new IstioRevision when you change the Istio.spec.version field. No other config changes  (e.g. when you change a value in Istio.spec.values) produce a new IstioRevision. Instead, they update the existing IstioRevision, just like when the InPlace strategy is used. 

We need to introduce a new update strategy that creates a new IstioRevision for any change to the config (either spec.version or spec.values.