Add _entities query to resolve interfaces to objects in the supergraph

[nicolerenee]

This adds the _entities Query which is used in graphql federation by the supergraph to resolve interfaces to concrete object types.

[nicolerenee]

Ran a version of this PR in compose-stack and it shows the query works that previously failed.

query {
  node(id: "lctnloc-sEdikPSFsjYJjJxssdTkY") {
    ... on Location {
      name
      owner {
        id
        ... on Tenant {
          name
        }
      }
    }
  }
}

Response:

{
  "data": {
    "node": {
      "name": "Just a test location",
      "owner": {
        "id": "tnntten-rXirlFQULBHDw9urtOjya",
        "name": "Development"
      }
    }
  }
}