Allow for setting of custom scopes.

I spoke with @jrb about this already, ticketing it.

A project that I am on at Kessel Run uses authservice for North/South Zero Trust authentication. When you couple this with an application (say for instance Grafana or ArgoCD) and it's own OAuth capability and Keycloak you are not able to specify custom scopes. The capability exists in the authservice application, but the helm chart doesn't support passing it down.