Expose project_id on /api/v4/pages/domains

What does this MR do?

This MR exposes pages domains's project_id.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

The !15160 (merged) MR adds an administrative endpoint to list all pages domains but doesn't exposes the project_id of the domains, which is required to modify the domains since you need it in domain API (PUT /projects/:id/pages/domains/:domain for example).

The API endpoint is now:

GET /pages/domains

curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/pages/domains

[
  {
    "domain": "ssl.domain.example",
    "url": "https://ssl.domain.example",
    "project_id": 1337,
    "certificate": {
      "expired": false,
      "expiration": "2020-04-12T14:32:00.000Z"
    }
  }
]

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

None