Allow predictors to make internal requests to other APIs

[deliahu]

Description

Currently, to make a request to a different Cortex API within a Predictor, it is necessary to access the other API's external endpoint. Kubernetes should be already managing DNS entries for all services, so it may be possible to reference another API by its service name, and allow DNS resolution to map it to the Kubernetes service (or virtual service). If the Istio gateway is skipped, it will be necessary to find an alternative to Istio's x-request-id header for tracking in-flight requests.