Fix federation orphan types

[paulpdaniels]

In federation it is possible that you declare a type which has no entry point in the graph but which is nonetheless required to be part of the outgoing schema. Consider:

// Schema A
@key(fields:"id")
type User {
  id: String!
}

type Query {
  me: User!
}

// Schema B
@key(fields:"id")
extends type User {
  id: String! @external
  team: Team!
}

type Query {
  team(id: String): Team
}

In the above graphs they should become federated with User gaining the team field through declaration, however currently the implementation ignores types which don't have entry points into the graph from Query so the users extension would be skipped.

This PR allows the schema to include these orphan types as long as there is an entity resolver defined for the typel.

[paulpdaniels]

We add these only here to avoid polluting the original graph since these new types don't have any utility since they couldn't be accessed through a normal query anyway.

[ghostdogpr]

Looks good!