Support jCasbin as the authorization backend

[veotax]

jCasbin is an authorization library that supports models like ACL, RBAC, ABAC.

Related to RBAC, casbin has several advantages:

1. roles can be cascaded, aka roles can have roles.
2. support resource roles, so users have their roles and resource have their roles too. role = group here.
3. the permission assignments (or policy in casbin's language) can be persisted in files or database.
4. multiple models like ACL, BLP, RBAC, ABAC, RESTful are supported.

And you can even customize your own access control model, for example, mix RBAC and ABAC together by using roles and attributes at the same time. It's very flexible.

I saw there's a group-based access control mechanism for ROR, but I think it can be extended to more powerful and flexible models like RBAC and ABAC through the integration of jCasbin. What do you think? Thanks.