Reference token support

[nover]

trafficstars

While working with #15 I discovered that it does not seem like this identity server integration has support for reference tokens?

I would really love to have that "back-channel" communication feature that IdentityServer4's middleware for ASP.netcore MVC provides, as it allows to call and get further claims based off the access token, it being JWT or Reference.

Do you have any pointers on where to start adding this into the code-base?

Access token validation middleware from IDP4:

https://github.com/IdentityServer/IdentityServer4.AccessTokenValidation