Create an endpoint for one external commerce provider. Where ClickFunnels generates the
signing secret, it is returned as webhook_secret in this response only, so save it
immediately: later reads return just webhook_secret_last_four. Where the provider issues
its own signing secret, the endpoint is created without one and you PATCH it in
afterwards.

Not every provider is available to every workspace: creating an endpoint for one a
workspace has not been granted returns 403.