feature: Virtual Resources: add validation for ResourceSchemaStorageVirtual in APIExport spec

[gman0]

Feature Description

VRs in APIExport's spec is missing validation.

Proposed Solution

We should check:

• CachedAPIs feature gate is enabled.
• ResourceSchemaStorageVirtual.Reference is valid.

Alternative Solutions

No response

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Additional Context

No response

[gman0]

ResourceSchemaStorageVirtual.IdentityHash is not trivially checked. APIExport.Spec.Resources[].Storage.Virtual has only EndpointSlice reference and we have no standard way to retrieve its parent object and/or its identity.