Add an endpoint for Keycloack OAuth2 demo

As shown in this iOS client repo
https://github.com/corinnekrych/KeycloakDemo
we will create aiOS cookbook for that see AGIOS-184

we need to have our own backend. Here we're relying on KC examples backend.
https://github.com/keycloak/keycloak/blob/master/examples/demo-template/README.md#step-6-traditional-oauth2-example