Getting Shipping rates through GraphQL/REST API for Carrier Services

[travis-ENL]

Hello,

I'm implementing an off-Shopify sales channel, and want to get a list of Shipping Rates for a shopping cart. It's possible to do this for stores that have ShippingZones configured, but I'm a little stumped at how I would do this for a store that has Carrier Services enabled, either through GraphQL or the REST API. 

The `carrier_services` endpoint only seems to return Carrier Services your app has created. If the store has connected their FedEx account - how can I get rates from this service?

Any suggestions? 

Thank you.