Introduce Istio.spec.components or equivalent

The upstream operator has a `spec.components` field that allows you to:

• enable/disable individual Istio components such as pilot, cni, ambient
• configure the namespace for each component
• override the container image hub an tag
• configure k8s-related fields for each component (resource requests/limits, affinity, etc.)

We should introduce a similar field in Istio and IstioRevision so that users can do (most of) what's described above. However, it might make sense to somehow incorporate these fields in Istio.spec.values, so that users can configure a component in a single place instead of two places (e.g. values and components). Or maybe even squash them directly into Istio.spec.