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

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.

The issue names no files or tests. Start by locating the existing admission policy CRD, raw Casbin model handling, and controller enforcer setup; then map how templates, parameters, and validation would fit together. Done means reusable built-in templates, parameterized CRD support, dynamic enforcer loading, and validation are implemented, but the issue provides no concrete acceptance tests.

Written by the indexing model from the issue text.