Have rover dev watch for subgraph add/delete in config option

[michael-watson]

Description

rover dev can currently uses the --supergraph-config option to point at a supergraph config file (docs). But if you add or remove a subgraph from the config while working in it, rover dev doesn't update as expected.

If a subgraph is added to the --supergraph-config during a rover dev session, no subgraph is added to the current rover dev session.

If a subgraph is deleted from the --supergraph-config during a rover dev session, the subgraph still exists in the current rover dev session.

Workaround

Restart rover dev:

• Kill process
• Rerun same terminal command

Use Cases

• Apollo Workbench 😁 - If a session is active and someone adds a subgraph, it would be nice to have this hot-reload handled in rover. I currently workaround this by stopping and restarting the dev session which is fine for smaller graphs, but maybe untenable for larger graphs with hundreds of subgraphs.