CRD Validation Rules for Required Cross-Resource Reference Fields

### What problem are you facing?

For a cross-resource reference field `spec.forProvider.x, we generate three fields:
1. A name reference field, e.g., `spec.forProvider.xRef`
2. A label selector field, e.g., `spec.forProvider.xSelector`
3. The reference target field `spec.forProvider.x`

Because any of these fields can be specified, upjet currently generates all of them as optional fields.

### How could Upjet help solve your problem?

We had better require at least one of these reference fields via [CRD validation rules](https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#validation-rules), if the reference target field is marked as required in the underlying Terraform schema.

The issue names no files, tests, or entry points. Start by locating generation of cross-resource reference fields and handling of required fields in the Terraform schema. Done means generated CRDs require at least one of the reference fields when the target is required, with validation coverage.

Written by the indexing model from the issue text.