[feature] integrate the default detector into Enforcer

[hsluoyz]

the default detector(s) should be enabled by default for Casbin enforcer.

the detector will auto run after all policy rules are loaded (like at the end of LoadPolicy).

Enforcer should also add RunDetections() method to explicitly run all detectors

Enforcer should also have SetDetector() and SetDetectors() to let user configure detectors. (the default detector(s) are set automatically and internally inside Casbin enforcer, if user doesn't explicitly call SetDetector() and SetDetectors())

there should be a test case to test enforcer + default detector, and detect the error in the enforcer's method