Integrate with Glewlwyd

See: https://github.com/babelouest/glewlwyd/issues/184#issuecomment-845783765

We should provide help and support including development efforts to help Glewlwyd integrate with Casbin. We need to communicate with them, know our customer's need and help them with the code.

I suggest putting this in a high priority (than the issues proposed by our own people) because we have been "working behind closed doors" for long (we raise issues by ourselves, then solve them) and this can be the first time for us to show Casbin-Cpp is also useful for others.