Re-evaluate whether IstioRevision.spec.values should be schemaless

Currently, the IstioRevision.spec.values field has no schema, since it's produced by combining Istio.spec.values and Istio.spec.rawValues. However, this prevents us from using CEL-based validation, since CEL only allows you to reference fields that appear in the schema.

Perhaps IstioRevision should have both values and rawValues, just like the Istio CR.