graph/subgraph misusage errors

[JakeDawkins]

• Add error for attempts to introspect a subgraph on graphs which don't support _service (i.e. not spec-compliant subgraphs)
• Add error for attempts to push a subgraph to a non-federated graph (maybe?)
• Add error for attempts to run subgraph check on a non-federated graph

Fixes #121

[EverlastingBugstopper]

What I'd love to see for rover is the following happy path, assuming mygraph is a federated graph

create a new subgraph: rover subgraph create mygraph --name my-new-subgraph --schema ./my-new-subgraph.graphql --routing-url https://my-new-subgraph.example.com/api/graphql

publish a new schema without updating the routing url: rover subgraph publish mygraph --name my-new-subgraph --schema ./my-new-subgraph.graphql

publish a new schema AND update the routing url rover subgraph publish mygraph --name my-new-subgraph --schema ./my-new-subgraph.graphql --routing-url https://my-new-endpoint.example.com/api/graphql

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If you tried to rover subgraph publish and the subgraph name did not exist, you'd get an error message pointing you to rover subgraph create

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If you tried to rover subgraph publish to a graph that exists but is not federated, you'd get an error and be told to create a new federated graph (could maybe have a way to override this if you Really Wanted To convert a graph from non-federated to federated, but imo this feels like a footgun)

[JakeDawkins]

@lrlna I think we agreed we wanted to add a flag to confirm the conversion from monolith to federated graph.

For monolithic graphs or variants with nothing pushed to it, we didn't want to force someone to confirm the change, since it's not destructive of anything. But for existing monoliths, we did want them to confirm with a flag --convert or something maybe?

We said this would take a quick roundtrip to the API to see if the graph was federated to begin with. Probably using the Query.graph.implementingServices, and checking to see if there are any and what kind they are (federated/non-federated).

I hope this makes sense :)