[Bug] unable to extend Casbin with custom role manager

[ghost]

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Describe the bug

The are several parts in the code where instead of calling the objects through the interface the object is being cast to a specific implementation.

Places where you can find the issue:

• https://github.com/casbin/casbin/blob/master/rbac_api.go#L311
• https://github.com/casbin/casbin/blob/master/rbac_api_test.go#L310
• https://github.com/casbin/casbin/blob/master/enforcer.go#L742
• https://github.com/casbin/casbin/blob/master/enforcer.go#L751

To Reproduce
Steps to reproduce the behavior:

1. Create an enforcer
2. Set the role manager with your custom implementation
3. Execute any function that goes through any of the above like GetNamedImplicitPermissionsForUser
4. See error: interface conversion: rbac.RoleManager is *autz.RoleManager, not *defaultrolemanager.RoleManager

Expected behavior

The function should be called through the interface

Desktop (please complete the following information):

• OS: macOS 12.6
• Casbin Version github.com/casbin/casbin/v2 v2.54.0

[casbin-bot]

@tangyang9464 @JalinWang

[JalinWang]

Thanks! Disentanglement definitely should be done. I'll try to make the interface more general.