Add The Idempotency-Key HTTP Header Field (#2202)

Close #2199, adding the suggested spec to the list.

### Changes to `index.json`
This update would trigger the following changes in `index.json`:

<details><summary>Add spec (1)</summary>

```json
{
  "url": "https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-idempotency-key-header/",
  "seriesComposition": "full",
  "shortname": "idempotency-key-header",
  "series": {
    "shortname": "idempotency-key-header",
    "currentSpecification": "idempotency-key-header",
    "title": "The Idempotency-Key HTTP Header Field",
    "shortTitle": "The Idempotency-Key HTTP Header Field",
    "nightlyUrl": "https://www.ietf.org/archive/id/draft-ietf-httpapi-idempotency-key-header-07.html"
  },
  "nightly": {
    "url": "https://www.ietf.org/archive/id/draft-ietf-httpapi-idempotency-key-header-07.html",
    "status": "Editor's Draft",
    "repository": "https://github.com/ietf-wg-httpapi/idempotency",
    "sourcePath": "draft-ietf-httpapi-idempotency-key-header.md",
    "alternateUrls": [],
    "filename": "draft-ietf-httpapi-idempotency-key-header-07.html"
  },
  "organization": "IETF",
  "groups": [
    {
      "name": "Building Blocks for HTTP APIs Working Group",
      "url": "https://datatracker.ietf.org/wg/httpapi/"
    }
  ],
  "title": "The Idempotency-Key HTTP Header Field",
  "source": "ietf",
  "shortTitle": "The Idempotency-Key HTTP Header Field",
  "categories": [
    "browser"
  ],
  "standing": "good"
}
```
</details>

### Tests
These changes look good! 😎

Co-authored-by: browser-specs-bot <>

Author: browser-specs-bot

specs.json

@@ -84,6 +84,13 @@
       "sourcePath": "draft-davidben-http-client-hint-reliability.md"
     }
   },
+  {
+    "nightly": {
+      "repository": "https://github.com/ietf-wg-httpapi/idempotency",
+      "sourcePath": "draft-ietf-httpapi-idempotency-key-header.md"
+    },
+    "url": "https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-idempotency-key-header/"
+  },
   "https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-layered-cookies/",
   "https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-no-vary-search",
   "https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-rfc6265bis",