Add label_key validator

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

Description:

• Validate Kubernetes label keys.

Acceptance criteria:

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