Add created_at to job webhooks

What does this MR do?

Related to #232887 (closed)

Tested using https://webhook.site/

Screenshots (strongly suggested)

Before

{
  "object_kind": "build",
  "ref": "root-master-patch-02048",
  "tag": false,
  "before_sha": "73e43273731cf0aa6fee618ef412572c34217d58",
  "sha": "012823f6a16195defbb74e716a17073a90ee5d7d",
  "build_id": 1031,
  "build_name": "dependency_scanning",
  "build_stage": "test",
  "build_status": "failed",
  "build_started_at": null,
  "build_finished_at": "2021-02-24T19:00:18.499Z",
  "build_duration": null,
  "build_allow_failure": true,
  "build_failure_reason": "stuck_or_timeout_failure",
  "pipeline_id": 63,
  "runner": null,
  "project_id": 2,
  "project_name": "Gitlab Org / Gitlab Shell",
  "user": {
    "id": 1,
    "name": "Administrator",
    "username": "root",
    "avatar_url": "http://127.0.0.1:3000/uploads/-/system/user/avatar/1/avatar.png",
    "email": "admin@example.com"
  },
  "commit": {
    "id": 63,
    "sha": "012823f6a16195defbb74e716a17073a90ee5d7d",
    "message": "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.",
    "author_name": "Administrator",
    "author_email": "admin@example.com",
    "author_url": "http://127.0.0.1:3000/root",
    "status": "failed",
    "duration": null,
    "started_at": null,
    "finished_at": "2021-02-24T19:00:30.391Z"
  },
  "repository": {
    "name": "Gitlab Shell",
    "url": "ssh://zamakkat@127.0.0.1:2222/gitlab-org/gitlab-shell.git",
    "description": "Sed velit voluptatem ut qui maiores id ut quae.",
    "homepage": "http://127.0.0.1:3000/gitlab-org/gitlab-shell",
    "git_http_url": "http://127.0.0.1:3000/gitlab-org/gitlab-shell.git",
    "git_ssh_url": "ssh://zamakkat@127.0.0.1:2222/gitlab-org/gitlab-shell.git",
    "visibility_level": 20
  },
  "environment": null
}

After

{
  "object_kind": "build",
  "ref": "root-master-patch-02048",
  "tag": false,
  "before_sha": "73e43273731cf0aa6fee618ef412572c34217d58",
  "sha": "012823f6a16195defbb74e716a17073a90ee5d7d",
  "build_id": 1031,
  "build_name": "dependency_scanning",
  "build_stage": "test",
  "build_status": "failed",
  "build_created_at": "2021-02-23T02:41:37.886Z",
  "build_started_at": null,
  "build_finished_at": "2021-02-24T19:00:18.499Z",
  "build_duration": null,
  "build_allow_failure": true,
  "build_failure_reason": "stuck_or_timeout_failure",
  "pipeline_id": 63,
  "runner": null,
  "project_id": 2,
  "project_name": "Gitlab Org / Gitlab Shell",
  "user": {
    "id": 1,
    "name": "Administrator",
    "username": "root",
    "avatar_url": "http://127.0.0.1:3000/uploads/-/system/user/avatar/1/avatar.png",
    "email": "admin@example.com"
  },
  "commit": {
    "id": 63,
    "sha": "012823f6a16195defbb74e716a17073a90ee5d7d",
    "message": "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.",
    "author_name": "Administrator",
    "author_email": "admin@example.com",
    "author_url": "http://127.0.0.1:3000/root",
    "status": "failed",
    "duration": null,
    "started_at": null,
    "finished_at": "2021-02-24T19:00:30.391Z"
  },
  "repository": {
    "name": "Gitlab Shell",
    "url": "ssh://zamakkat@127.0.0.1:2222/gitlab-org/gitlab-shell.git",
    "description": "Sed velit voluptatem ut qui maiores id ut quae.",
    "homepage": "http://127.0.0.1:3000/gitlab-org/gitlab-shell",
    "git_http_url": "http://127.0.0.1:3000/gitlab-org/gitlab-shell.git",
    "git_ssh_url": "ssh://zamakkat@127.0.0.1:2222/gitlab-org/gitlab-shell.git",
    "visibility_level": 20
  },
  "environment": null
}

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