rover dev should accept a <GRAPH_REF> argument

[EverlastingBugstopper]

Situation

rover dev exists, and requires that you run rover dev once for each individual subgraph.

Problem

Managing multiple terminals quickly gets out of hand, and most times you want to make one small change to one or two subgraphs instead of the whole thing at once. This means that the development story as is becomes cumbersome quickly.

Proposed Solution

If you specify a <GRAPH_REF> along with rover dev, then Rover should "extend" the supergraph that lives in Studio.

Proposed Implementation

Any time a graph ref is passed to rover dev

1. Check if the graph is federated, if so, fetch all subgraphs (name, url, schema), if not, just fetch the schema and prompt for the name and url
2. Use the subgraph definitions from the graph ref as a base, and sub in local subgraph definitions as they are supplied by the user
3. Download internal build plugins as opposed to open source build plugins in order to ensure the composed supergraph schema matches what would be provided in managed federation

Attaching additional rover dev sessions should still work, but we should make sure that specifying the same <GRAPH_REF> in a second command does not fetch all of those subgraphs again if the leader session already has knowledge of them.

Open Questions

You can add more subgraphs by running rover dev more times. Should we require that you can only specify one <GRAPH_REF>? Or should we allow you to dev multiple <GRAPH_REF>s together?