Add `"x-octokit": { "diff": { ... } }` to keep track of differences between versions

[gr2m]

Part of octokit/octokit-next.js#15

update all the generated ghes-3.0.json (etc) OpenAPI files to include a "diff" key in "x-octokit": {}. The "diff" object is only present if there are any changes. All ghes-* versions other than the latest one should include a diff to the next GHES version, the rest should have a diff to github.com. Example:

{
  "diff": {
    "github.com": {
      "type": "added" // "added" | "removed" | "changed"
    }
  }
}