feat: add expiresAt field to PolicyException for expiration support

[JagjeevanAK]

Summary

This PR adds an optional expiresAt field to PolicyException to support time-based expiration of policy exceptions for CEL-based policies.

Changes

• Add expiresAt field (metav1.Time) to PolicyExceptionSpec in all API versions (v1, v1alpha1, v1beta1)
• Add IsExpired() method to PolicyException to check if the exception has expired
• Update CRDs and generated code

Related Issues

• Related to: [Feature] PolicyException should have optional expiration date and additional metadata kyverno#14491
• Related PR: chore: Added optional expiration date and additional metadata to PolicyException kyverno#14539

Usage

apiVersion: policies.kyverno.io/v1beta1
kind: PolicyException
metadata:
  name: temporary-exception
spec:
  policyRefs:
  - name: my-policy
    kind: ValidatingPolicy
  matchConditions:
  - name: match-specific-resource
    expression: "object.metadata.name == 'test-resource'"
  # Exception expires on Jan 1, 2025
  expiresAt: "2025-01-01T00:00:00Z"

Once the expiresAt time has passed, the exception will no longer be applied to incoming requests.