fun fetchAuthProvider(onComplete: (Either<AuthProvider, SippoResourceError>) -> Unit): Unit
Fetches the available AuthProvider.
onComplete - Callback that will be invoked when the request is completed.
It will receive the available provider in case of success or a SippoResourceError otherwise.
It is possible that the error may be caused by the fact that previous server versions only support
the Provider model. In that case, fetchProviders method should be called instead.