Add k8s_resource_name validator

Implement a new provider::validatefx::k8s_resource_name validator.

Description:

• Validate Kubernetes resource names (DNS-1123 compliant, lowercase, max 63 chars).

Acceptance criteria:

1. Subtask: Implement k8s_resource_name validator in internal/validators
2. Subtask: Add unit tests for k8s_resource_name validator
3. Subtask: Add k8s_resource_name function to internal/functions package
4. Subtask: Add unit tests for k8s_resource_name function
5. Subtask: Register k8s_resource_name in registry.go
6. Subtask: Add documentation in examples/functions/k8s_resource_name/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf