feat(templates): implement policy library for reusable casbin models

[AKonnyaku]

In existing implementation, users currently need to manually define raw casbin models in every policy. To simplify this process the following updates are proposed:

• creating a built-in library for common templates like pod security would be beneficial;
• the admission policy CRD should also be refactored to accept parameters for these templates;
• in addition the controller needs a loading mechanism to instantiate enforcers dynamically;
• providing robust validation logic ensures user parameters match the selected template schema;
• ultimately these changes lower the barrier to entry through out-of-the-box usability.