Best way for a spawner to call a service e.g. during start()?

[rcthomas]

Is there a proper way for a Spawner, say in the course of start(), to make a call to a JupyterHub service that uses the HubAuthenticated mixin?

The idea would be to dynamically determine settings that are managed through another service. This seems related to #964, but this boils down to how to get ahold of the user's API token to make an API call from a Spawner method.