Add @interfaceObject and @composeDirective at Federation 2 directive lists.

[wangmir]

This is just an update of PR #2699

This PR excludes the federation 2 directives added in #2635 from being added to DirectiveRoot. Without this exclusion, usage of @interfaceObject or @composeDirective fails at runtime with something like:

{
  "data": null,
  "errors": [
    {
      "message": "directive interfaceObject is not implemented",
      "path": [
        "startTest",
        "currentQuestion"
      ]
    }
  ],
  "extensions": {
    "valueCompletion": [
      {
        "message": "Cannot return null for non-nullable field Mutation.startTest",
        "path": []
      }
    ]
  }
}

---

Currently, when using @interfaceObject, the GraphQL schema generates the directive template in the auto-generated file because those directives are not included in the list of federation directives, and so are not excluded from the code generation.

So, it causes a "not implemented" error when using @interfaceObject even though this directive is a federation 2.3 directive.

This commit just adds those directives to the list.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 75.897% (-0.2%) from 76.092% when pulling 0adf3e7 on wangmir:fix_interface_object into 3ff523b on 99designs:master.

[StevenACoffman]

Thanks!