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