Support Customization of CasbinRule class

[wakemaster39]

Right now, CasbinRule has its own declarative_base that is hard coded down in all levels of the code.

Situations come up in use where you want to have your own class for various reasons such as a single declarative base. Rather than forcing a copy and pasting of the entire adapter and making your own. Allow setting the class yourself and all other logic renames usable.