Allow users to authenticate via a trusted assertion (JWT or similar)

[nhoughto]

Username / password is a good simple start to authenticating proxy users, but a method that scales with less configuration change would be more convenient especially in a world where #18 is a reality. If I can push dynamic config when being able to authenticate clients more dynamically too would be a great fast-follower.

[nhoughto]

could use kube service accounts if this worked, wouldn't be a transparent kube-aware type of thing but could allow clients to auth without hard coded creds if they wanted to invest in the integration.