Add an AssertionHandler to the middleware

[kevcoxe]

I have been using this repo in addition to itzg/saml-auth-proxy but I wanted to add some extra functionality where on login take the assertion from the SAML provider and add RBAC to Grafana.

To do this I added a AssertionHandler to samlsp middleware. Inside of the ServeACS function before creating the session I call the AssertionHandler.HandleAssertion function and give it the assertion.

This way you can create your own AssertionHandler and have it do whatever you want, in my case add RBAC to opensource Grafana.

Let me know if there is something I should change. I am currently using a fork for my purpose but would love to see this added upstream.