[RRFC] Add an icon url field

Add a icon url field to the package.json spec, which would be returned in the search results.

Edit: This link would be available in the search API results.

So, using the example result from a search below, a field would be returned in the package object:

{
  "objects": [
    {
      "downloads": {
        "monthly": 17877,
        "weekly": 3863
      },
      "dependents": "177",
      "updated": "2025-12-22T14:06:11.084Z",
      "searchScore": 294.53653,
      "package": {
        "name": "homebridge-lib",
        "keywords": [
          "homekit",
          "homebridge"
        ],
        "version": "7.2.0",
        "description": "Library for homebridge plugins",
        "sanitized_name": "homebridge-lib",
        "icon": "https://github.com/ebaauw/homebridge-lib/path/to/icon_file.png",
        "publisher": {
          "email": "erik.baauw@xs4all.nl",
          "username": "ebaauw"
        },
        "maintainers": [
          {
            "email": "erik.baauw@xs4all.nl",
            "username": "ebaauw"
          }
        ],
        "license": "Apache-2.0",
        "date": "2025-12-08T13:57:14.187Z",
        "links": {
          "homepage": "https://github.com/ebaauw/homebridge-lib#readme",
          "repository": "git+https://github.com/ebaauw/homebridge-lib.git",
          "bugs": "https://github.com/ebaauw/homebridge-lib/issues",
          "npm": "https://www.npmjs.com/package/homebridge-lib"
        }
      },
      "score": {
        "final": 294.53653,
        "detail": {
          "popularity": 1,
          "quality": 1,
          "maintenance": 1
        }
      },
      "flags": {
        "insecure": 0
      }
    }
  ],
  "total": 1,
  "time": "2025-12-25T20:42:48.227Z"
}

Start by reviewing the package.json specification and the search API result shape described in the issue. Determine how an icon URL should be represented in package metadata and returned within each package object. Done means the proposal has an agreed field definition and the search results expose it consistently.