As we already clear any local sessions before redirecting to the
Identity Provider, and the callback always redirects to a pre-configured URL,
there isn't really any need to maintain and verify state in the logout
callback.
In other words, the logout callback handler is simply a redirect handler.
There's a bunch of changes here, but in essence:
- split out openid configuration
- separate openid configuration between client/rp and provider
- consolidate client and provider related code in separate packages
These changes allow for simplification of the Handler, as well as a
bunch of test/mock code as the configuration is now instantiated
seperately from the client/provider code.
