Add support for client authentication

Some providers require the client itself to be registered to be allowed to access it. It's optional in the OIDC protocol but it's required by providers like Google for example.

Most of the work should probably be in https://github.com/xwiki-contrib/oidc/blob/master/oidc-authenticator/src/main/java/org/xwiki/contrib/oidc/auth/internal/endpoint/CallbackOIDCEndpoint.java#L124