Expose iid in pipelines api

What does this MR do and why?

Resolves #342223 (closed)

Adds iid to the pipelines endpoint, as per the linked issue.

• Related to !67365 (merged) (Adding iid to UI)
• Related to !76591 (closed) (Removing iid from docs because not exposed)

Screenshots or screen recordings

• Before:

  {
      "id": 221,
      "project_id": 21,
      "sha": "00d019fe52f7fb47cf0408401d376b3aa70a906b",
      "ref": "master",
      "status": "success",
      "source": "web",
      "created_at": "2021-09-27T06:33:29.695Z",
      "updated_at": "2021-09-27T06:34:02.991Z",
      "web_url": "http://127.0.0.1:3000/test-group/test-project/-/pipelines/221"
  },

• After:

  {
      "id": 221,
      "iid": 85,
      "project_id": 21,
      "sha": "00d019fe52f7fb47cf0408401d376b3aa70a906b",
      "ref": "master",
      "status": "success",
      "source": "web",
      "created_at": "2021-09-27T06:33:29.695Z",
      "updated_at": "2021-09-27T06:34:02.991Z",
      "web_url": "http://127.0.0.1:3000/test-group/test-project/-/pipelines/221"
  },

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.