- Allow a plugin to provide a settings form.
- Inject settings into the object, stop using static methods, simplify method signatures.
- Rename sendAuthenticationRequest() to authorize(), to match the oauth2 standard.
- Rename retrieveIDToken to retrieveIdToken to match coding standard.
- Create an OpenIDConnectClientBase abstract class.
- Move endpoint urls to the class, to allow a generic plugin to supply them
  from settings in the future.
- Introduce openid_connect_get_client(), openid_connect_get_plugin().