[6.x] Forms 2: Expose form sections, pages and access restrictions in the REST & GraphQL APIs

[duncanmcclean]

This pull request implements support for form sections in the REST API, and pages in both the REST and GraphQL APIs. The GraphQL API has had sections since #11466 but the REST API never got them, and neither API exposed pages.

sections returns every section across all pages, so it's the easy way to render a single-page form with the same structure as the Control Panel. pages returns each page's id, display, instructions and its sections, for rendering multi-page forms. Single-page forms return a single page.

This PR also fixes the GraphQL sections field only returning the first page's sections on multi-page forms.

Access restrictions

This PR also exposes require_login and restriction_message alongside the existing status, so headless frontends can tell whether a form is currently accepting submissions and what to show when it isn't.

restriction_message only covers the closed and limit reached cases. The login case is deliberately left out, since the API can't know who is visiting a headless frontend. require_login is exposed as a plain boolean instead so the frontend can make that call itself.

REST API

GET /api/forms/contact

{
  "data": {
    "handle": "contact",
    "title": "Contact",
    "fields": { "name": {...}, "email": {...} },
    "sections": [
      { "display": "Your Details", "instructions": null, "fields": { "name": {...}, "email": {...} } }
    ],
    "pages": [
      { "id": "main", "display": "Page 1 of 1", "instructions": null, "sections": [...] }
    ],
    "status": "open",
    "require_login": false,
    "restriction_message": null,
    "api_url": "http://example.com/api/forms/contact"
  }
}

GraphQL

{
    form(handle: "contact") {
        status
        require_login
        restriction_message
        pages {
            id
            display
            instructions
            sections {
                display
                instructions
                fields { handle display }
            }
        }
    }
}

---

Closes statamic/ideas#1320