Studio Uplink integration

[abernix]

Requirements

Uplink is the Studio machinery / GraphQL endpoint which provides supergraphs to running Gateways (today). We need to offer the Studio Uplink behavior in the Router, as well.

This very plausible builds on as a component of the umbrella #66.

The mechanism for retrieving it should match that of the Apollo Gateway, however, the packaging may be abstracted a bit differently, in particular because we may want to allow a Rover command (e.g., rover supergraph fetch [graphRef] --uplink to fetch from Uplink as well (as opposed to fetching from non-Uplink, which it does by default). Furthermore, it's possible that we want to offer the ability to externalize this from the Router by having it as a sidecar/agent.

Since part of this requires making a GraphQL request — something which Rover does already, one of the constraints might be matching the patterns that Rover uses (its GraphQL client, etc.). This should be investigated

Subtasks

Someone should own breaking these down into subtasks.

• Document existing Studio Uplink behavior from Gateway
• Design implementation for Studio Uplink in Router
• Implement Studio Uplink behavior as Rust Crate