[feature] add Casbin's Provider interface

the provider is the way to provide identity info for Casbin, including users, roles, user-role-mappings, permissions, role-permission-mappings, etc.

it extends the role manager interface, because role manager only handles user-role-mappings, but provider handles all.

a provider can also be viewed as a way to import other auth permissions into Casbin.

the provider implementations can be cloud providers (like AWS, Azure, GCP) or identity vendors (like Okta, Auth0) or auth languages (like XACML)

this issue will first add the Go interface of Provider to Casbin codebase