Provide extra data to page extensions

The future Universal Pixel extension needs more data besides the page URL. Let's provide that data already to page extensions, so the Universal Pixel extension will be easier to build.

The extra data needed is page uuid, sitemap item uuid and channel id. So a full JSON blob provided to BR_EXTENSION.onContextChanged() could be:

{
  context: 'page',
  data: {
    channelId: "example-preview",
    pageId: "e7ac2a2d-c3b5-48b0-80d8-abf3aa583ee4",
    pageUrl: "https://www.example.com/news/page.html",
    sitemapItemId: "afc202fd-03c6-44ff-b4fc-d08ce812bb34",
  } 
}

The pageId resembles the page meta-data property 'HST-Page-Id'.

The sitemapItemId resembles the page meta-data property 'HST-SitemapItem-Id'