add AuthState via reducer

[pamapa]

something like:

export interface AuthState {
  user?: User;
  isAuthenticated: boolean;
  isLoading: boolean;
  error?: Error;
}

inside the context:

const [state, dispatch] = useReducer(reducer, initialAuthState);

Additionally a AuthReducer.tsx file is handling the reducing. I could put together a merge request, iff wanted.

[simenandre]

Thanks for opening this issue, @pamapa. I'm closing this now, but we'll use this for reference in regards to #652.

For reference, @pamapa have made a separate library that adds this. So, if you are looking for such functionality, have a look at: https://github.com/pamapa/react-oidc-context