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. BTW, https://github.com/vmware/dispatch already uses the Golang's Casbin. And jCasbin keeps exactly the same API and advantages as Golang Casbin.

I saw there's an Authentication and Authorization Design, it discusses about roles on users, roles on resources, tenants, RESTful. And I think jCasbin is a good choice for these requirements. And jCasbin provides more than that, including regex match, three kinds of key match, RBAC role hierarchy, ABAC, etc. What do you think? Thanks.