fix: Skip federation2 directives generation

[rlch]

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": []
      }
    ]
  }
}

Please let me know if there's anything to do for this PR. I couldn't find documentation on how to regenerate testdata/, unsure if go generate ./... was sufficient. Thanks for reviewing + for this great package (and hello from Melbourne)!

I have:

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

[coveralls]

coverage: 75.418% (-3.8%) from 79.196% when pulling cb497f2 on rlch:master into 470fca8 on 99designs:master.

[StevenACoffman]

Please do this:

go generate ./...; cd _examples; go generate ./...

[dariuszkuc]

looks like this was merged in #2821, should this be closed?

[StevenACoffman]

thanks!