Make jwks_uri configurable

We would like to be able to serve the well-known endpoint and the jwks data from a different domain (to work around cookie-blockers).  Proxying the existing file from a different domain *should* Just Work, except that the jwks-endpoint has the OIDCng-servername hardcoded.  Can we make that configurable?

The other endpoints (authz, token, userinfo, introspect, etc) should not change.