new: subgraph health links and buffer for sdk & router.

[sanchaymittal]

[rhlsthrm]

I think it's better to make this a mapping. Like we can use:

"subgraphMap": {
  "subgraph-url": "subgraph-index-node-url"
}

If we use a different name we can leave the old key in there until we are done with it.

[sanchaymittal]

So instead of using root URI, do we use full Uri for mapping?