Add reconciliation interval to ImagePolicy

[makkes]

The introduction of the .spec.digestReflectionPolicy field makes it necessary for the ImagePolicyReconciler to regularly reconcile ImagePolicies so that it discovers updated images independently of its accompanying ImageRepository.

Some more context on this issue can be found in this comment thread.

ImagePolicy should receive a new .spec.interval field so that users can specify at which interval the ImagePolicy should be reconciled independently of the ImageRepository.