api: add HTTP endpoint for compiled discovery chain for external integrations

[banks]

We should probably/maybe have an HTTP version of the “compiled discovery chain” RPC for 1.6.0. So far everything Envoy can do can at least in theory be done by any other proxy or native app via our API. Concrete use-cases even beyond other sidecar proxy parity - integrating Ambassador or other north-south configs with things like routing/splitting config.

We'd need to review the graph/compile output and decide if we are confident enough to expose it as it is though as it becomes harder to mutate once we put it in the API.