`rover dev` should provide an endpoint to fetch subgraph SDLs

rover dev spins up a router which embeds sandbox. this sandbox endpoint does not have knowledge of all of the subgraph SDLs, only the API schema provided by the router. this is great for running queries since it's what clients will do, but it's not great for navigability in sandbox itself. the checks page is weird and you can't see all of the different subgraphs in the UI (even though we have UI for displaying multiple subgraph SDLs).

rover dev should spin up a proxy in front of the router, normal requests will pass through to the router, and requests to http://localhost:3000/subgraphs should return a JSON blob of all the subgraphs and their SDL like so:

{
  "subgraphs": {
    "subgraph-a": "subgraph-a SDL",
    "subgraph-b": "subgraph-b SDL",
  } 
}

apollographql / rover

`rover dev` should provide an endpoint to fetch subgraph SDLs #1269