Hi Robert,
custom components with config flow seem to be possible now (See https://github.com/home-assistant/home-assistant/pull/24946)
However, if I move your withings folder into config/custom_components there is no withings Integration which I could set up.
Update: Actually, the integration is there, just the name is not visible. Typing withings in the integrations search brings up one single item without a name. Clicking on it shows the profile I set up in configuration.yaml.
Update2: Working fine as custom_component, thanks Robert! Just wondering why the integration name is not shown.
Here is what I mean:
