Conformance: Add test for x-gateway-destination-endpoint-served #1670
Description
recently the EPP protocol was enhanced with additional required field.
Gateways should report back using metadata field which endpoint served the request.
More information here:
https://github.com/kubernetes-sigs/gateway-api-inference-extension/tree/main/docs/proposals/004-endpoint-picker-protocol#destination-endpoint-served
in order to pass the conformance test, the Gateways need to implement it (from what I know in GKE it's already implemented).
cc: @danehans @keithmattix @Xunzhuo @delavet