[InferenceModel update] Created fairness/identity header flag #1245
Description
This is related to: https://github.com/kubernetes-sigs/gateway-api-inference-extension/tree/main/docs/proposals/1199-inferencemodel-api-evolution#structure-change
Specifically: The EPP will expose a flag to define the header key (default: x-gateway-inference-fairness-id) that will be used in tracking Request Usage (which will act as the identifier for simple fairness implementation)
This will flag into the Flow Controller to be consumed there.